Structure determination, electronic and optical properties of rubidium holmium polyphosphate RbHo(PO 3 ) 4 Jing Zhu ⇑ , Hui Chen, Yude Wang, Hongtao Guan, Xuechun Xiao Department of Material Science and Engineering, Yunnan University, Kunming 650091, People’s Republic of China article info Article history: Received 11 January 2012 Received in revised form 11 April 2012 Accepted 11 April 2012 Available online 20 April 2012 Keywords: Polyphosphate RbHo(PO 3 ) 4 Crystal structure Optical properties abstract Structural, optical, and electronic properties of a new alkali metal-rare earth polyphosphate, RbHo(PO 3 ) 4 , have been investigated by means of single-crystal X-ray diffraction, power X-ray diffraction, elemental analysis, and spectral measurement. RbHo(PO 3 ) 4 crystallizes in the monoclinic with space group P2 1 /n and Z = 4. It is described as a three-dimensional (3D) architecture built up of PO 4 double spiral chains and HoO 8 polyhedra by corner-sharing. The 11-coordinated rubidium atoms are located in infinite tun- nels. Additionally, in order to gain further insight into the relationship between property and structure of RbHo(PO 3 ) 4 , theoretical calculation based on the density functional theory (DFT) was performed using the total-energy code CASTEP. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Solid-state alkali metal-rare earth phosphate materials have been extensively studied for their structural diversity [1,2] and high luminescence efficiency [3,4]. Furthermore, their chemical and thermal stability ensures the feasibility of the industrial appli- cation. In the system material, the common basic structural unit is PO 4 group, and the varied condensation of PO 4 groups results in several structural families with the general formula M I LnP 2 O 7 [5], M I Ln(PO 3 ) 4 [6], and M I 3 LnðPO 4 Þ 2 [7] (M I = alkali metal, Ln = rare earth metal), etc. Recently, to our knowledge, structures and opti- cal properties of some polyphosphates MLn(PO 3 ) 4 (M = Li, Na, K, Ln = La, Eu, Tb, Ho, Yb, Y, Dy) [8–13] were successively reported. We have also made efforts to synthesize MLn(PO 3 ) 4 (M = K, Cs, Ln = La, Ce, Eu) [14–16]. Maksimova and co-workers explored the structure of polyphosphate containing holmium in combination with rubidium in the MLn(PO 3 ) 4 family by power X-ray diffraction data [17]. However, the electronic and optical properties have not been reported. In order to find new material with good property and enrich this family of compounds, we successfully synthesized the solid-state polyphosphate crystal RbHo(PO 3 ) 4 by the high temperature solution reaction in our laboratory. In the present paper, we will report the crystal structure of rubidium holmium polyphosphate RbHo(PO 3 ) 4 based on the single-crystal X-ray diffractometer data for the first time. Further- more, we will explore the electronic and optical properties. The calculation of crystal energy band, density of states (DOSs), and population analysis is carried out by the density functional theory (DFT) method in order to analyze the chemical bonding property and electronic origin of optical transition. 2. Experimental 2.1. Synthesis Single crystals RbHo(PO 3 ) 4 were grown by the high temperature solution reaction. All reagents were purchased commercially and used without further purification. The starting materials contain- ing analytical reagents RbNO 3 , Ho 2 O 3 , and NH 4 H 2 PO 4 were weighed in the molar ration of Rb/Ho/P = 14/1/24 and finely ground in an agate mortar to ensure the best homogeneity and reactivity, then placed in a corundum crucible and preheated at 373 K for 6 h. Afterwards, the sinter was reground and heated to 673 K for 24 h. Finally, the melt was cooled to 423 K at a rate of 6 K/h and air-quenched to room temperature. A few colorless block-shaped crystals were obtained from the product. Polycrystalline sample of RbHo(PO 3 ) 4 was synthesized by the solid-state reaction of stoichiometric amounts (Rb/Ho/P = 1:1:4) of analytical reagents RbNO 3 , Ho 2 O 3 , and NH 4 H 2 PO 4 . The pulverous mixture was allowed to react at 973 K for 100 h with several inter- mediate grindings in an opening corundum crucible. The white powder product was obtained. 2.2. Crystal structure determination by X-ray diffractometer A single crystal of RbHo(PO 3 ) 4 with approximate dimensions of 0.15 0.11 0.09 mm 3 was selected for indexing and intensity 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.04.033 ⇑ Corresponding author. Tel./fax: +86 871 5031124. E-mail addresses: [email protected], [email protected](J. Zhu). Journal of Molecular Structure 1030 (2012) 204–208 Contents lists available at SciVerse ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Transcript
Journal of Molecular Structure 1030 (2012) 204–208
Contents lists available at SciVerse ScienceDirect
Structure determination, electronic and optical properties of rubidium holmiumpolyphosphate RbHo(PO3)4
Jing Zhu ⇑, Hui Chen, Yude Wang, Hongtao Guan, Xuechun XiaoDepartment of Material Science and Engineering, Yunnan University, Kunming 650091, People’s Republic of China
a r t i c l e i n f o
Article history:Received 11 January 2012Received in revised form 11 April 2012Accepted 11 April 2012Available online 20 April 2012
Keywords:PolyphosphateRbHo(PO3)4
Crystal structureOptical properties
0022-2860/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.molstruc.2012.04.033
Structural, optical, and electronic properties of a new alkali metal-rare earth polyphosphate, RbHo(PO3)4,have been investigated by means of single-crystal X-ray diffraction, power X-ray diffraction, elementalanalysis, and spectral measurement. RbHo(PO3)4 crystallizes in the monoclinic with space group P21/nand Z = 4. It is described as a three-dimensional (3D) architecture built up of PO4 double spiral chainsand HoO8 polyhedra by corner-sharing. The 11-coordinated rubidium atoms are located in infinite tun-nels. Additionally, in order to gain further insight into the relationship between property and structureof RbHo(PO3)4, theoretical calculation based on the density functional theory (DFT) was performed usingthe total-energy code CASTEP.
� 2012 Elsevier B.V. All rights reserved.
1. Introduction
Solid-state alkali metal-rare earth phosphate materials havebeen extensively studied for their structural diversity [1,2] andhigh luminescence efficiency [3,4]. Furthermore, their chemicaland thermal stability ensures the feasibility of the industrial appli-cation. In the system material, the common basic structural unit isPO4 group, and the varied condensation of PO4 groups results inseveral structural families with the general formula MILnP2O7 [5],MILn(PO3)4 [6], and MI
3LnðPO4Þ2 [7] (MI = alkali metal, Ln = rareearth metal), etc. Recently, to our knowledge, structures and opti-cal properties of some polyphosphates MLn(PO3)4 (M = Li, Na, K,Ln = La, Eu, Tb, Ho, Yb, Y, Dy) [8–13] were successively reported.We have also made efforts to synthesize MLn(PO3)4 (M = K, Cs,Ln = La, Ce, Eu) [14–16]. Maksimova and co-workers explored thestructure of polyphosphate containing holmium in combinationwith rubidium in the MLn(PO3)4 family by power X-ray diffractiondata [17]. However, the electronic and optical properties have notbeen reported. In order to find new material with good propertyand enrich this family of compounds, we successfully synthesizedthe solid-state polyphosphate crystal RbHo(PO3)4 by the hightemperature solution reaction in our laboratory.
In the present paper, we will report the crystal structure ofrubidium holmium polyphosphate RbHo(PO3)4 based on thesingle-crystal X-ray diffractometer data for the first time. Further-more, we will explore the electronic and optical properties. Thecalculation of crystal energy band, density of states (DOSs), and
ll rights reserved.
.hotmail.com (J. Zhu).
population analysis is carried out by the density functional theory(DFT) method in order to analyze the chemical bonding propertyand electronic origin of optical transition.
2. Experimental
2.1. Synthesis
Single crystals RbHo(PO3)4 were grown by the high temperaturesolution reaction. All reagents were purchased commercially andused without further purification. The starting materials contain-ing analytical reagents RbNO3, Ho2O3, and NH4H2PO4 wereweighed in the molar ration of Rb/Ho/P = 14/1/24 and finelyground in an agate mortar to ensure the best homogeneity andreactivity, then placed in a corundum crucible and preheated at373 K for 6 h. Afterwards, the sinter was reground and heated to673 K for 24 h. Finally, the melt was cooled to 423 K at a rate of6 K/h and air-quenched to room temperature. A few colorlessblock-shaped crystals were obtained from the product.
Polycrystalline sample of RbHo(PO3)4 was synthesized by thesolid-state reaction of stoichiometric amounts (Rb/Ho/P = 1:1:4)of analytical reagents RbNO3, Ho2O3, and NH4H2PO4. The pulverousmixture was allowed to react at 973 K for 100 h with several inter-mediate grindings in an opening corundum crucible. The whitepowder product was obtained.
2.2. Crystal structure determination by X-ray diffractometer
A single crystal of RbHo(PO3)4 with approximate dimensions of0.15 � 0.11 � 0.09 mm3 was selected for indexing and intensity
a Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
J. Zhu et al. / Journal of Molecular Structure 1030 (2012) 204–208 205
data collection on a Bruker APEX-II CCD diffractometer withgraphite-monochromated MoKa radiation (k = 0.71073 Å) usingthe x/2h scan mode at the temperature of 296 K. An empiricalabsorption correction was applied using SADABS program. Thecrystal structure of RbHo(PO3)4 was solved using direct methodand refined on F2 by full-matrix least-squares method with theSHELXL97 program package [18]. The position of the Ho atomwas refined by the application of the direct method, and theremaining atoms were located in succeeding difference Fouriersynthesis. Then, all atoms were refined with anisotropic thermalparameters. The final refined solution was checked with theADDSYM algorithm in the program PLATON [19], and no highersymmetry was found. In order to confirm the chemical composi-tion of the compound, single crystal RbHo(PO3)4 investigated onthe diffractometer was analyzed by Energy-dispersive X-ray spec-trometry (EDX) using a QUANTA200 scanning electron microscope.The obtained result is in good agreement with that obtained by therefinement of the crystal structure. No impurity elements havebeen detected. Crystallographic data and structural refinementfor RbHo(PO3)4 are summarized in Table 1. The atomic coordinatesand thermal parameters are listed in Table 2. Selected bond lengthsand angles are given in Table 3. Further detail on crystallographicstudy and elemental analysis is given as Supporting material.
2.3. Computational detail
The crystallographic data of RbHo(PO3)4 determined by single-crystal X-ray diffraction was used to calculate the electronic bandstructure and density of states. The calculation was performed withthe density functional theory (DFT) using a nonlocal gradient-cor-rected exchange–correlation functional (GGA-PBE) and performedwith the CASTEP code [20,21], which uses a plane wave basis setfor the valence electrons and norm-conserving pseudopotential[22] for the core states. Hubbard U correction is employed to treatthe strong correlation f electrons of the Ho3+ ion, and LDA + Uscheme was processed with CASTEP code. Hubbard U was set bythe default value (6.0 eV for the valence f shell of Ho atoms) ofCASTEP code [20]. The number of plane waves included in the basiswas determined by a cutoff energy Ec of 400 eV. Pseudo atomic
Table 1Crystallographic data and structural refinement for RbHo(PO3)4.
Formula RbHo(PO3)4
Formula weight (g mol�1) 566.28Temp(K) 296(2)Wavelength (Å) 0.71073Crystal system MonoclinicSpace group P21/nUnit cell dimensionsa (Å) 10.2649(5)b (Å) 8.8564(5)c (Å) 10.9435 (6)b (�) 106.210(1)V (Å3); Z 955.32(9); 4Dcalc (g cm�3) 3.803l(mm�1) 13.09F(000) 1016Crystal size (mm) 0.15 � 0.11 � 0.09h range (�) 2.41–30.11Limiting indices �14 6 h 6 14; �12 6 k 6 12;
�15 6 l 6 14Reflections collected 10327Independent reflections 2736Refinement method Full-matrix least-squares on F2
GOF 1.000Final R indices (I > 2r(I)) R1 = 0.0372, wR2 = 0.0766R indices (all data) R1 = 0.0439, wR2 = 0.0793Largest diff. peak and hole
(e��3)3.238 and �2.6088
calculations were performed for O – 2s22p4, P – 3s23p3, Rb –4s24p65s1, and Ho – 4f115s25p66s2. The calculating parameters andconvergent criterions were set by the default value of CASTEP code[20].
2.4. Spectral measurement
The sample used for spectral measurement is polycrystallinepowder synthesized by the solid-state reaction. To give evidencethat it contains pure phase of sample, we determined the powderX-ray Diffraction (XRD) pattern of RbHo(PO3)4 using a RIGAKUDMAX-3B diffractometer with CuKa radiation (step size of 0.02�and range 2h = 10–90�). The XRD pattern for the powder sampleis in good agreement with the pattern based on single-crystalX-ray solution in position, indicating the phase purity of the as-synthesized sample of RbHo(PO3)4. The experimental and simu-lated X-ray powder diffraction patterns of RbHo(PO3)4 are givenas Supporting material. The absorption spectrum was recordedon a HITACHI U-4100 UV/VIS/NIR spectrophotometer in the wave-length range of 200–1000 nm. The photoluminescence (PL) mea-surement was carried out on a HITACHI F-4500 fluorescencespectrophotometer using Xe lamp as the excitation source at roomtemperature.
3. Results and discussion
3.1. Structural description
Rubidium holmium polyphosphate crystal RbHo(PO3)4, whichwas synthesized by the high temperature solution method, crystal-lizes in the monoclinic space group P21/n with four formula unitsper unit cell, as clearly shown in crystallographic data (Table 1).The crystallographically distinct atoms of the asymmetric unit inthe structure are one rubidium, one holmium, four phosphorus,and twelve oxygen atoms (Table 2). The crystal structure forRbHo(PO3)4 is drawn in Fig. 1. It is described as a three-dimen-sional (3D) framework made up from PO4 double spiral chainsand Ho-polyhedra, which is isostructural with CsEu(PO3)4 [14]we have previously synthesized. The rubidium atoms are locatedin infinite tunnels along a axis delimited by the 3D framework.As illustrated in Fig. 2, the phosphorus atom is four-coordinated,and crystallographically distinct P(1)O4, P(2)O4, P(3)O4, andP(4)O4 tetrahedra form PO4 double spiral chains by corner-sharing.
Table 3Selected bond distances (Å) and angles (�) for RbHo(PO3)4.
Symmetry codes: (i) �0.5 + x, 0.5 � y, �0.5 + z; (ii) 1.5 � x, 0.5 + y, 1.5 � z; (iii) 1 � x, �y, 1 � z; (iv) 1 � x, �y, 2 � z; (v) 0.5 + x, �0.5 � y, 0.5 + z; (vi) 0.5 + x, 0.5 � y, 0.5 + z; (vii)1.5 � x, �0.5 + y, 2.5 � z; (viii) 1.5 � x, �0.5 + y, 1.5 � z; (ix) �0.5 + x, �0.5 � y, �0.5 + z; (x) x, 1 + y, z; (xi) 1.5 � x, 0.5 + y, 2.5 � z; (xii) x, �1 + y, z.
Fig. 1. View the three-dimensional structure of RbHo(PO3)4 along a axis (Polyhe-dron represents PO4 tetrahedron).
Fig. 2. View PO4 double spiral chains (the Rb and Ho atoms are omitted for clarity).
Fig. 3. The coordinated environment of the holmium cation.
206 J. Zhu et al. / Journal of Molecular Structure 1030 (2012) 204–208
The P–O bond distances and O–P–O bond angles in Table 3 showsthat there is the best severe deformation in P(1)O4 tetrahedron.There are eight PO4 tetrahedra as the repeating unit in the doublespiral chains, which are linked with Ho-polyhedra. The holmiumcation is eight-coordinated with the Ho–O bond distances rangingfrom 2.316(3) to 2.439(3) Å (Table 3), which are consistent withthose reported previously [8]. HoO8 polyhedron is corner-andface-connected with two and two Rb-polyhedra (Fig. 3), respec-tively, and corner-shared with neighboring two PO4 tetrahedra.The isolation of HoO8 polyhedra gives rise to the large Ho–Ho
distances, the shortest of which is 6.3844(4) Å. To our knowledge,such large Ho–Ho distances have been not reported in the M IHo(-PO3)4 family [8,9,23], which can possibly avoid concentration fluo-rescence quenching [24,25]. So, luminescent performance ofRbHo(PO3)4 is worth expecting. The rubidium cation located inthe intersecting tunnel is surrounded by eleven oxygen atoms,and the Rb–O bond distances with the large range of 2.917(4)–3.479(4) Å (Table 3) implies that RbO11 polyhedra are severely dis-torted. Neighboring two RbO11 polyhedra are connected by corner-sharing (Fig. 4).
3.2. Band structure and density of states
The energy band structure and density of states (DOSs) ofRbHo(PO3)4 were calculated with the DFT method. The calculatedband structure along high symmetry points of the first Brillouinzone is illustrated in Fig. 5, where the labeled k-points are presentas Z (0.0, 0.0, 0.5), G (0.0, 0.0, 0.0), Y (0.0, 0.5, 0.0), A (�0.5, 0.5, 0.0),B (�0.5, 0.0, 0.0), D (�0.5, 0.0, 0.5), E (�0.5, 0.5, 0.5), and C (0.0, 0.5,0.5). The top of valence bands (VBs) is close to the Fermi level(0.0 eV), and the highest energy (0.0 eV) is localized at the Y point.The bottom of conduction bands (CBs) is nearly flat, and the lowest
Fig. 4. The coordinated environment of the rubidium cation.
Fig. 5. Calculated band structure of RbHo(PO3)4.
Fig. 6. Total and partial densities of states of RbHo(PO3)4.
Fig. 7. The absorption spectrum of RbHo(PO3)4.
J. Zhu et al. / Journal of Molecular Structure 1030 (2012) 204–208 207
energy (5.28 eV) is localized at the G point. Accordingly, it isindicated that RbHo(PO3)4 shows an insulator character with anindirect band gap of around 5.28 eV. In order to assign these bands,total density of states (TDOSs) and partial DOS (PDOS) are plottedin Fig. 6. According to Fig. 6, the bands ranging from �2.5 eV to theFermi level can be mainly assigned as the O – 2p and Ho – 4f states.The O – 2p, Rb – 4p, Ho – 4f, P – 3p, and P – 3s states give rise to theVBs ranging from �12.5 eV to the Fermi level. The O – 2s, Ho – 5p,Rb – 4s, P – 3p, and P – 3s states result in the VBs between �22.7and �17.0 eV. The VBs from �47.5 to �44.0 eV result from theHo – 6s states. In addition, the CBs between 4.5 and 7.0 eV aremostly formed by the Ho – 4f and P – 3p states.
Furthermore, we also elucidate the feature of chemical bondingfrom the nature of TDOS and PDOS (Fig. 6). We observe that thedensity of the O – 2p state (64 electrons/eV) is more than one ofthe P – 3p state (5.5 electrons/eV) between �12.5 and 0.0 eV. Thisresult shows that some electrons in P – 3p transform into the VBsand take part in the interactions between P and O atoms. Accord-ingly, the hybridization between P – 3p and O – 2p sates takesplace and covalent bond character appears between P and O atoms.It is also found that the PDOS of Rb – 5s states is mainly located inCBs, which determine that the Rb–O bond is ionic. The chemicalbonding properties are also evident from the population analysis.The calculated bond orders of the P–O, Ho–O, and Rb–O bond are0.44–0.82, 0.23–0.31, and 0.0 e in a unit cell (covalent single bondorder is generally 1.0 e), respectively. It is indicated that the Ho–Obonds with longer distances (2.316(3)–2.439(3) Å) have muchsmaller bond orders than the P–O bonds ranging from 1.476(4)to 1.614(4) Å. Accordingly, we can deduce that the covalent charac-ter of the P–O bond is larger than that of the Ho–O bond, and theionic character of the Rb–O bond is larger than that of the Ho–Obond in RbHo(PO3)4.
3.3. Luminescent properties
Luminescent properties of polycrystalline power sample forRbHo(PO3)4 were explored. Fig. 7 illustrates the absorption spec-trum of RbHo(PO3)4. The absorption edge is around 300 nm(4.14 eV). Comparing the calculated result (about 5.28 eV) of thecrystal with the experimental one (about 4.14 eV) of the powdersample shows that the transmission width is larger for the crystalthan the powder sample, which indicates that the result of the cal-culation is reasonable. The strong absorption peak appears ataround 250 nm (4.97 eV). According to our calculated DOS(Fig. 6), the peak is assigned as the electronic 4f–4f transitions ofthe Ho3+ ion.
The emission spectrum of RbHo(PO3)4 at room temperature ex-cited with 350 nm is shown in Fig. 8. There are six blue–green lightemissions around 464, 482, 494, 507, 522, and 556 nm in the visi-ble region, which result from the intraconfigurational electronictransitions of the Ho3+ ion [26–28]. The strongest emission peak
Fig. 8. The emission spectrum of RbHo(PO3)4 (excited at 350 nm).
208 J. Zhu et al. / Journal of Molecular Structure 1030 (2012) 204–208
located at the wavelength of 464 nm and the emission bandsaround 482, 494, and 507 nm are mainly caused by the 5F3 ?
5I8
transition. The emissions at 522 and 556 nm are attributed to the5F4 ?
5I8 and 5S2 ? 5I8 transitions, respectively.
4. Conclusions
In summary, rubidium holmium polyphosphate crystal RbHo(-PO3)4 has been synthesized by the high temperature solution reac-tion. The crystallographic data obtained by single-crystal X-raydiffraction shows that RbHo(PO3)4 belongs to the monoclinic withspace group P21/n and Z = 4. PO4 double spiral chains and HoO8
polyhedra form a 3D architecture by corner-sharing, and the 11-coordinated rubidium atoms are located in infinite tunnels.According to the calculated population analysis, the covalent char-acter of the P–O bond and the ionic character of the Rb–O bond arestrongest. The absorption spectrum and the calculated band struc-ture shows that RbHo(PO3)4 is an indirect band-gap insulator. Theemission spectrum indicates that RbHo(PO3)4 exhibits broad emis-sion in the blue–green light region.
Acknowledgments
This investigation was based on work supported by the NationalNatural Science Foundation of China (No. 20901066), Training pro-gram for young academic and technical leader in Yunnan Province,Training program for young teacher in Yunnan University, and Pro-gram for Innovative Research Team (in Science and Technology) inUniversity of Yunnan Province.
Appendix A. Supplementary material
Full results of element analysis and experimental and simulatedX-ray powder diffraction patterns of RbHo(PO3)4 are available(PDF). Further details (CIF) on the crystal structure investigationare obtained from the Fachinformationszentrum Karlsruhe,76344 Eggenstein-Leopoldshafen, Germany, (fax:+497247808666;e-mail: [email protected]) on quoting the depository num-ber CSD-423965). Supplementary data associated with this articlecan be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.04.033.
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