1. Introduction

High pressure is very effective tool for research of discovering new phenomena and of exploring newmaterials. For example, some materials undergosuperconducting transition under high pressure. Also,it is well known that some insulators become metallicat high pressure. Conversely, there are some singularmaterials in which metallic state becomes unstable for insulating state at high pressure. Changes in thesephysical properties are induced by the changes in theatomic positions originated from the changes in thevolume at high pressure.

Recently, much attention has been drawn topressure effects on physical properties of materials.Such as the study of pressure induced phase transition, the pressure effect on the superconducting transitiontemperature of high-Tc oxide superconductors, thechange in electronic properties of the heavy fermionsystem at high pressure, and the peculiar phenomenaof organic conductors under high pressure areinteresting topics. Thus, it has been required for theapparatus to realize the highly reliable measurementsof physical properties at low temperatures under highpressure. Especially, to clarify the origin of suchchanges in the physical properties, structuralrefinement at low temperatures under high pressure isan important research project.

In order to examine the physical properties at high pressure, it is, of course, desirable to apply thepressure as high as possible to the sample. However,one cannot simply assert the higher, the better. This

remark may sound strange, but this means in a caseusing the pressure as the tool for elucidating thephysical properties that the physical quantities shouldbe investigated in such pressure regions where nostructural change occurs. That is, the pressure iseffective for tuning the electronic structure. Besides,the pressure uniformity is important for theexperiments.

Among conventional high pressure apparatus, the piston-cylinder system is excellent in terms ofhydrostatic conditions although its maximal pressureis no more than 3 GPa or so. This type is easy tohandle and is widely used for studies of the physicalproperties at low temperatures. With this type,however, it is difficult to conduct crystallographicstudy like X-ray diffractometry. On the other hand,the diamond anvil cell being optically transparent cangenerate pressures beyond 100 GPa and therefore itsapplication is extended to structure analysis, infraredspectroscopy, Brillouin scattering, etc. But this type is inferior in the uniformity of pressure and, moreover,the quantity of sample it can accommodate is verysmall. Because of such limitations, the diamond anvilcell is not suitable for quantitative evaluation of otherphysical quantities, such as electric and magneticproperties.

High pressure apparatus of cubic anvil type has so far been used for various researches under pressure up to around 10 Gpa, for example, structural studiesabove room temperatures obtained by X-raydiffraction and studies of the pressure effect on thephysical properties at low temperature obtained by the

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The Rigaku Journal

Vol. 15/ number 2/ 1998

CONTRIBUTED PAPERS

HIGH PRESSURE APPARATUS FOR IN SITU X-RAYDIFFRACTION AND ELECTRICAL RESISTANCEMEASUREMENT AT LOW TEMPERATURETAKEHIKO MATSUMOTO*, JIE TANG* AND NOBUO M]RI**

* National Research Institute for Metals, Tsukuba, lbaraki 305, Japan

** Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan

A new high pressure apparatus with cubic anvil device has been developed to carry out X-raydiffraction and electrical resistance measurement simultaneously up to 10 Gpa at low temperaturedown to 5 K. This apparatus consists mainly of a uniaxial press for pressure generation, an X-raydiffraction system with energy dispersive solid state detector and a cryostat. We show the typicalresults obtained by this apparatus.

electric measurements such as electrical resistance.But as compared with the measurement of electricalresistance, it is far more difficult to carry out X-raydiffraction experiments with the cubic anvil typeapparatus at low temperature under high pressure.Hence no reports have been published yet on suchwork. In the structural study at low temperature underhigh pressure, the high quality results will be obtainedby using neutron diffraction in the pressure regionbelow about 1 Gpa [1]. But there are no establishedmeasurement techniques yet to deal with higherpressure regions than 1 GPa. For studies of thepressure effects, especially in the case that changes inphysical property are observed at low temperatures, itis essential to examine whether or not a structuralchange occurs. From this point of view, we worked onthe development of a high pressure apparatus withcubic anvil press which should permit X-raydiffraction experiments and electrical resistancemeasurement simultaneously at low temperature.

With the cubic anvil press, the amount of samplecan be 3 to 5 orders of magnitude greater in volumethan with the diamond anvil cell. This merit enablesmeasurement of the physical properties with aprecision comparable to that under ambientconditions. Thus far its major application has been X-ray diffraction experiments at high temperature underhigh pressure with a built-in heater by takingadvantage of the large capacity for sample. This time,we attempted to stuff an X-ray diffraction sample intothe upper gasket part, as will be explained later, whileinstalling a sample for electrical resistancemeasurement into the lower part. In this way wechallenged simultaneous measurements of X-raydiffraction and electrical resistance at low temperature under high pressure.

2.System Configuration and Characteristics

Our cubic anvil type cryogenic high pressuregeneration system is based, as the prototype, on thelow temperature and high pressure apparatus [2] being operated at the Institute for Solid State Physics, theUniversity of Tokyo. Some new design attempts areincorporated specifically for low temperature X-raydiffraction experiments. This apparatus is made up ofa uniaxial type 250-ton press supported by twocolumns, an X-ray diffraction system, and anadiabatic type vacuum cryostat which accommodatesa cubic anvil for high pressure generation (Fig. 1). The cubic anvil section was initially designed formeasurement of electrical properties in a temperature

range from room temperature to low temperature(liquid helium temperature) under high pressures of10 GPa in maximum and of 9 GPa for routineoperation. At the same time it was aimed atmeasurement of lattice constant with the sameprecision as at room temperature and atmosphericpressure.

With a newly developed optical control system,the apparatus has made it possible to conduct X-raydiffractometry in low temperature/ high pressureregions, so far regarded as difficult techniques.Further, a newly prepared low temperaturecontrolling device has made it possible to upgradephysical property studies that excel in quality andreliability compared with earlier studies usingconventional systems. The characteristics of eachcomponent will be discussed in the followingsections.2. 1. Pressure Generation

The cubic anvil device is so designed that agasket as a pressure medium is compressed evenlyfrom six directions with six anvil tops, therebyproducing a hydrostatic high pressure for a samplesealed in the gasket (Fig. 2). Two of the six anvil topsare directly fixed to retaining dies arranged at upperand lower part of a sample section, respectively. Inoperation these upper and lower anvils are movedcloser to each other by a certain distance, and thisdisplacement will cause the other horizontallyarranged four anvils to slide on a 45° slope in theretaining dies, respectively, by the same distancetoward the center. This means that a uni-axial press

Vol. 15 No. 2 1998 26

Fig. 1 Photograph of apparatus

drives all anvils evenly at the same time (Fig. 3). Theanvil top is made of tungsten carbide and the retainingdie is Ni-Cr-Mo steel. A slit is worked in each die bytaking into account an X-ray diffraction experimentunder high pressure at low temperature. The gasketmaterial is a mixture of amorphous boron and epoxyresin in 4:1 weight ratio, both of which arecharacterized by less X-ray absorption.

To serve as the pressure system it is necessary totransmit loads adiabatically to the anvil tops at lowtemperature. To this purpose, the thick plates made offiber reinforced plastics (FRP) are used, as shown inFig. 4 of a cross-sectional construction. As shown inthis figure, the cubic anvil device is placed betweenboth ends of pressure transmitting FRP column. In the operational condition, because there exists thetemperature gradient from room temperature outsideof cryostat to the liquid helium temperature at thesample position, the sample pressure will vary due tothe change in the load caused by the thermal expan-sion/contraction of FRP column with increasing/de-creasing temperature. As a counter-measure, the oilpressure system driving the uniaxial press is sodesigned that a change in load is automatically corr-ected and that a constant pressure is always applied tothe sample section despite temperature changes.2.2. Cryostat

By using the cubic anvil device, we planned to doexperiments for physical properties over a tempera-ture range from low to room temperatures at highpressure, such as the lattice constants, the electricalresistance and the change in magnetization. For theseexperiments, a cryostat is required to control the

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Fig. 2 Schematic illustration of sample sealed in thegasket. The sample is pressurized from 6 direction byusing a cubic anvil press.

Fig. 3 Cross section of the cubic anvil.

Fig. 4 Cross section of the cyrostat.

temperature of the cubic anvil device consisting of theretaining dies and the six anvils, and the gasket at thecenter of which a sample is sealed.

To cool down such large device to the liquidhelium temperature, the following construction isgenerally required. That is, an adiabatic container forliquid nitrogen should be arranged as the outermostcomponent. Then another adiabatic container forliquid helium should be placed inside. Further, a cubicanvil device for high pressure generation should beaccommodated in the latter adiabatic container. Now,if measurement concerns only the physical propertieslike the electric and magnetic properties, it is possibleto conduct it in a virtually equilibrium state withoutconsidering special temperature control. This isbecause the heat capacity of what is contained inside is so large that temperature changes are relatively slowenough compared with the duration of time requiredfor each measurement at the respective temperatures.Whereas, if an X-ray experiment should be carried out concurrently, there may occur a need to hold at eachmeasuring temperature for several hours or more inorder to get a diffraction pattern with a good signal-to-noise ratio. For the temperature control, therefore, adesirable construction is that adiabatic space shouldbe set up in a liquid helium environment so that thehigh pressure generator may be installed in that space.This is not easy in practice because of restrictionscoming from costs and the overall dimensions. Toresolve this problem, we decided to employ a thermalconduction system designed to cool the cubic anvildevice indirectly by the flow of refrigerants, such asliquid nitrogen and liquid helium. That is, there is nostorage for liquid gases inside the cryostat. Instead, aheat exchanger is mounted to each retaining die, andoperated by the flow of liquid nitrogen fortemperatures down to 80 K and subsequently liquidhelium for temperatures lower than that. As a result,just a single stainless steel sheet can serve as the outerwall of the cryostat to hold the inner vacuum. Thus,despite the provision of a radiation shield, a spacearound the dies is enough to prepare the sample forexperiment. Besides, although we were initiallyconcerned about if we could attain 10 K or so, thetemperature proved to be 5 K in a preliminaryexperiment without load and turned out to be below 7K in an actual experiment. We intend to aim at an even lower temperature sometime in future. To realize it,however, we must expect some trial-and-errorprocess.

As mentioned previously, the load is transmittedto the anvil tops from the press head via the FRPcolumn. Under the operation in this configuration, toprotect the cryostat from unexpected stresses, theFRP column is mechanically separated from thecryostat by means of bellows. Our design is also suchthat by using this bellows, the position of the cryostatcan be controlled independently against the positionof sample section which moves due to the pressureand temperature changes.2.3.In situ X-ray System

In X-ray experiment at high pressure, it is notallowable to use much quantities of sample for thecondition keeping the homogeneity of the appliedpressure. It is, therefore, less than approx. 2 cubicmillimeters. In addition, the gasket (pressuremedium), etc surround the sample. Due to suchrestrictive conditions, the intensity of the resultingdiffraction pattern is fairly lower than that obtainableby ordinary X-ray diffraction experiments. This iswhy an X-ray diffraction experiment under highpressure is difficult; hence it will be desirable toprepare a powerful X-ray source. For the X-raysource, we use a separate type (tabletop) tungstenrotating anode (the maximum output: 60 kV-300 mA) assembly made by Rigaku Corporation, and theenergy dispersive method is applied for the X-raydetection by using a solid state detector (SSD).

The X-ray optical system is composed of theaforementioned separate type X-ray source, anincident collimator, a receiving collimator, and anSSD mounted on the goniometer. An installing stagefor X-ray source is mounted on an X-ray goniometertable, and equipped with adjusting mechanisms forthree directions. That is, X-Y-Z directions (X. parallel to the X-ray beam, Y. horizontal and perpendicular toX, Z. vertical), The X-ray goniometer table with theX-Y-Z stage is installed to the main body of the pressframe. This makes it easy to adjust the X-ray opticalaxis. For the Z-direction adjustment of thegoniometer table, an inverter is used allowing remote-control operation and automatic control (to bementioned later). The incident and scattered X-raycollimators are installed to the cryostat via thebellows, but they are mechanically isolated from thecryostat to also allow fine adjustment for the opticalalignment.

In this X-ray diffraction experiment, the incidentX-ray beam passes through a narrow gap between theflanks of the anvil tops and impinges on a sample

Vol. 15 No. 2 1998 28

stuffed in the gasket (also serving as a pressuremedium). The resultant scattered X-rays are countedby the GE-SSD (Fig. 5). For the liquid nitrogencontainer to cool the Ge detector, a 7.5 liters largecapacity one is employed with consideration given tothe possibility of a long experiment time over severaldays.

Now, as the pressure rises, the anvil gap becomesgradually narrower. In addition, as shown in Fig. 4,the press head (ram) is arranged at the bottom of pressframe, and the top of FRP column is fixed to the topbolster of press frame. For the reason, when the load is applied from the bottom, the position of the samplesection will also gradually rise due to a reduction inthe gap and the compression of the FRP.Consequently, there occurs a deviation in the relativeposition of the Z-direction between the X-ray beamand sample making it difficult to obtain an X-raydiffraction pattern. To cope with this, a positionaldetector is installed to the cryostat and it monitors achange in the sample position. Upon receiving itsoutput, the goniometer table is driven by the invertermotor. In this way, an adjustment can be made at alltimes to keep constant the relative position betweenthe X-ray beam and sample (Fig. 6).

2.4.Simultaneous Measurement of ElectricalResistance with in situ X-ray Diffraction

A 4-terminal method is employed for electricalresistance measurement. For measurement of thepressure effect on electrical resistance, we conductedit in similar way [2,3], which had already beenexecuted at the Institute for Solid State Physics, theUniversity of Tokyo. Further, when an X-raydiffraction experiment should be made concurrently,the configuration of a sample section is slightlymodified. The following is our present method, which we have attained after repeating trial and error.

Because a set of anvil tops with 6 mm on edge ofsquare face is available for enough sample space, weused it for simultaneous measurement of X-raydiffraction and electrical resistance. As a gasket, thecube of boron-epoxy is used; each edge of which is 8mm long. In practice, the gasket was divided into twopieces, upper part with 4.5 mm high (4.5x8x8 mm)and lower part with 3.5 mm high (3.5x8x8 mm). A 2mm dia. hole was made in the upper part to use as asample chamber for X-ray measurement. Likewise, a3 mm dia. hole was made in the lower part and aTeflon inner cell for electrical resistancemeasurement was inserted into the hole. Figure 7

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Fig. 5 Configuration of gasket and samples: sample forX-ray experiment sealed in the upper part, sample forelectrical resistance measurement mounted in lowerpart. Path of X-ray beam is also shown.

Fig. 6 Scheme of position controller.

shows the gasket as a pressure medium and the sample configuration.

For electrical resistance measurement, Au wire of20 microns diameter is used as lead wire for a sample.It is attached to the sample surface with gold paintcontact. Then the lead wire attached sample is inserted into the Teflon cell filled with Fluorinert liquid as apressure medium. At the outside of the cell, the leadwire is connected with gold foil (50 micron thicknessX500 microns width) before coming out to the squareface of anvil top. The lead wire, four in total, iselectrically connected to each of the four anvil topsarranged horizontally. On this occasion, the fouranvils are electrically insulated from the retaining dies by means of Teflon sheet and FRP sheets. At the timeof actual measurement, in order to make correction for the origin of a digital voltmeter and for thethermoelectromotive force at the sample, polarityinversion was made about the electric-current.

3.Measurement of Pressure and Temper-ature

The pressure generated on this apparatus wascalibrated by the change in the lattice constant ofNaCl, the equation of state of which is well known. Sofar it has been confirmed at room temperature that theanvil tops with a 4mm edge of square face and a 6 mmone can respectively generate 12 GPa (9 GPa for aregular use; the anvil top was damaged at 12 GPa) and6 GPa. Figure 8 shows a pressure calibration curve.

The pressure at low temperature is generallycalibrated by the pressure dependence ofsuperconducting transition temperature of bismuth,etc. But X-ray diffraction at low temperature isavailable in our apparatus, so the pressure calibrationcan be made more accurately with respect to theoptional temperature region.

For the sample temperature, it is normally best tomeasure it with a thermometer or thermocoupleplaced next to a sample. In the present case, however,measurement in such a way is extremely difficult dueto the small sample chamber under high pressure andthe absence of a suitable thermometer being valuableinside the small chamber at high pressure. Therefore a platinum palladium resistance thermometer insertedinto a copper holder is installed to the upper anvil ofthe apparatus, where is the closest place to the sample. Both electrical resistance and temperaturemeasurements are automatically performed on acomputer.

4. Experimental Example−− X-ray in situ Observation and ElectricalResistance Measurement of TmTe [4,5]

The Tm monochalcogenides with a NaClstructure display various physical propertiesdepending upon the difference in chalcogen, Te, Se or S. TmTe is a magnetic semiconductor. Its valenceband is formed by a filled p band consisting of the 5porbits of Te, while a vacant d band consisting of the

Vol. 15 No. 2 1998 30

Fig. 7 Arrangement of gasket for simultaneousmeasurement of X-ray diffraction and electricalresistance.

Fig. 8 Calibration curves of pressure for the two kinds ofanvil tops as a functin of the output of the uniaxial press.

5d orbits of Tm becomes a conduction band, and the4f13 level exists in an energy gap between them. AsTm is divalent ion, TmTe has a magnetic moment. Inother Tm chalcogenides, TmSe is a valencefluctuation material where Tm has an intermediatevalence between divalence and trivalence. Further,TmS in which Tm ion is trivalent shows a metallicconductivity with Kondo-like behavior in its electrical resistance. Variation in terms of Tm valence isbelieved to originate in the different lattice constants,resulting in the change in the electronic properties.TmTe has a largest lattice constant in chalcogenidesmentioned above. Accordingly, one can expect toinduce similar variations in terms of valence andphysical properties by applying pressure to TmTe.

The results of X-ray in situ observation andelectrical resistance measurement for TmTe areshown in Fig. 9(a), (b). It has been shown from the X-ray diffraction measurement that there are threeregions of variation about the pressure dependence ofthe volume change, as seen in Fig. 9(a) and an inset init (compressibility of TmTe obtained by differenti-ating (a)). In the low pressure region, the change of the

volume follows the compressibility of Tm2+, asindicated by dotted line. Thus a simple pressure effectmay be presumed. In the high pressure region above 2GPa, the volume reduction rapidly increases to comeclose to the compression curve of Tm3+. This isconsidered to be due to a change in Tm valence fromdivalence to trivalence. In the further higher pressureregion beyond 6 GPa, the pressure dependence of thevolume becomes smaller again, and new diffractionlines due to the structure phase transition are observed in a region exceeding 8 GPa, (F i g. 10).

The pressure dependence of the electricalresistance was measured at room temperature. As aresult, three regions of variation were also foundcorresponding with the change in the volume, asshown in Fig. 9(b). In the low pressure region, theelectrical resistance decreases exponentially withincreasing pressure, indicating the closing process ofan energy gap which existed between the 4f13 level ofTm and the conduction band. In the intermediatepressure region, the pressure dependence of theelectrical resistance becomes smaller considerably.Under pressures beyond 2 GPa, this rapid changeseems to be attributed to the vanishing of the energygap. In a further region of high pressure, the electricalresistance decreases in a discontinuous manner. It isconsidered to be due to the structure phase transition.

As a typical example, Fig. 11(a) shows thetemperature dependence of the electrical resistance at

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Fig. 9 Experimental results obtained from in-situ X-ray diffraction and electrical resistance at roomtemperature. (A) Pressure dependeces of volume, the inset shows compressibility. (B) pressure dependeceof electrical resistivity.

Fig. 10 X-ray spectra observed at various pressuresand at a room temperature.

5.3 GPa and 8 GPa. The temperature dependence in an intermediate region of 5.3 GPa is complicated.However the result of X-ray diffraction experiment inthis region at low temperature shows that thetemperature dependence of the lattice constantdecreases monotonically as indicated in Fig. 11 (b). Inother words, it is now made clear that the abnormalbehavior of the electrical resistance at lowtemperature is not directly connected to the structurephase transition. From these results through X-ray insitu observation and electrical resistance measure-ment, we have been able to clarify the valence state ofTmTe under high pressure. We have also obtainedvery interesting results about the metal-insulatortransition of NiSx, and CuIr2 (S1-xSex)4. We would liketo report on them in next chance.

5. Concluding Remarks

In this paper we have introduced a multipurposeX-ray diffraction apparatus which permits simul-taneous measurements of X-ray diffraction and

electrical resistance at low temperature under highpressure. At the present stage we can generate amaximum pressure of 12 GPa (normally up to 9 GPa). We can also get low temperatures down to 5K bynewly developed cooling techniques. The simul-taneous measurement enables one to obtain informa-tion on the physical properties and structure under thesame conditions of pressure and temperature. In thissense the system offers many advantages not only, for example, enhancing the reliability of the outcome butproviding data on which to decide whether the causeof a physical property change is a peculiarity of theelectronic state or a structure phase transition.

Since this is our first attempt which has just beenput into practice, we are aware there remain things tobe improved in some way or another. Nevertheless, as we are now fairly experienced in operation, webelieve our techniques can display high performancein the material research field for the future. Moreconcretely, we are aiming at generating a higherpressure region by improving the anvil top as well asat getting low temperatures below 4 K by pumpingthe liquid helium. As for the detector, we are planning to execute an angle-dispersive method for X-rayexperiment by use of an imaging plate and the like.

Acknowledgment

We wish to thank Mr. T. Kosaka, Institute ofApplied Physics, University of Tsukuba for helpfulcooperation extended to our experiments. We wouldalso like to express our appreciation to Prof. T. Suzuki and Dr. T. Matsumura, Faculty of Science, TohokuUniversity for providing us with a TmTe sample.

References[1] H. Takahashi, J. D. Jorgensen, B. A. Hunter, R. L.

Hitterman, Shiyou Pei, F. lzumi, Y. Shimokawa, K. Kubo and Manako, Physica C191 (1992) 248.

[2] N. M^ri, H. Takahashi and Y Miyane, Solid State Physics 25 (1990) 185 (in Japanese).

[3] N. M^ri and H. Takahasi, Proc. IV Intnat'l Conf. on HighPressure in Semiconductor Physics, Thessaloniki,1990, p. 152; idem, J. High Pressure Institute of Japan28 (1990) 124 (in Japanese).

[4] J. Tang, T. Kosaka, T. Matsumura, T. Matsumoto N. M6ri and T. Suzuki, Solid State Commun. 100 (1996) 571.

[5] T. Matsumura, T. Kosaka, J. Tang, T. Matsumoto, H.Takahashi, N. M6ri and T. Suzuki, Phys. Rev. Letter 78(1997)1138.

Vol. 15 No. 2 1998 32

Fig. 11 Experimental results obtained from in-situ X-ray diffraction and electrical resistance as a functionof temperature at typical pressures of 5.3 and 8 Gpa.(a) Temperature dependences of electrical resistivity.(b) Temperature dependences of lattice constant.

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