High Pressure Studies On The Double Tungstates NaBi（WO₄）₂and Its System Of Rare Earths Doped

The double tungstates MIMIII(WO4)2(MI=alkali metal，MIII=Trivalent element)are the derivatives of CaWO4. The MIand MIIIions, distributed in the position of Ca2+,make the double tungstate crystals a typical disordered structure. The system ofNaBi(WO4)2and its rare-earth doped compound is a kind of inorganic scintillationcrystal with excellent properties and important applications. The double tungstateNaBi(WO4)2as a new kind of matrix of rare-earth laser crystal has attracted the publicattention. A great many studies have been conducted on NaBi(WO4)2crystals atambient conditions, including the synthesis and growth process of the crystal, thestructural characterization, anisotropic linear optical properties (e. g. refractiveindices), IR absorption and Raman scattering, the ability of scintillation, nonlinearoptical properties (e. g. Raman shift), up-conversion, etc. NaBi(WO4)2is determinedas a tetragonal structure of I41/a (No.88) symmetry (Z=2) at ambient conditions,which is crystallized in the melt of tetragonal scheelite CaWO4. The Na+and Bi3+cations are distributed randomly at the4s Wyckoff positons in local region, althoughthey are of short-range order. The double tungstate with the doping of rare-earthcations R3+could be unsed as active laser mediμm and its optical properties could beclipped. The spectra of the compounds with the doping of rare-earth cations R3+rangefrom ultraviolet to mid-IR. Another feature of these system is the matrix materialscould be doped with high concentration.So far, high-pressure studies of tungstate AWO4(A=Ca, Sr, Ba, Pb, Eu) have been carried out and certain universal laws (e.g. the relationship between ionic radiusand phase transition pressure, the relationship between phase transition sequence andcoordination nμmber, the conditions of the pressure-induced amorphization, etc.) havebeen concluded by researchers. However, few studies have quantitatively assessed thehigh-pressure properties of the matters. In addition, the optical properties of therare-earth doped NaBi(WO4)2at ambient conditions have been widely studied, but thestudies on the high-pressure structural stability, which is related to the characteristicsof lμminescence, have not been reported. Since NaBi (WO4)2is the derivative ofCaWO4, it can be expected that the high-pressure behaviors of NaBi (WO4)2might beas rich as tungstate AWO4. Therefore, the studies on high-pressure structural phasetransition and spectroscopic properties of NaBi(WO4)2and its rare-earth dopedcompounds, are significant to the theoretical research and practical applications.In order to further understand the laws of the structural phase transitions and thespectral properties of the double tungstate with the typical disordered crystal structureunder high pressure,we have conducted a systematic high-pressure study onNaBi(WO4)2and the rare-earth doped Re: NaBi(WO4)2(Re=Ce, Nd, Er, Yb) throughhigh pressure synchrotron radiation angle dispersive X-ray diffraction (ADXRD)technology and high-pressure Raman spectroscopy. The laws of the structural phasetransitions and the scattering spectral variations of this system under high pressurehave been conclude as follows:1. The high-pressure studies on structural phase transition and the scatteringspectra have been performed on NaBi(WO4)2. At7.57GPa, the NaBi(WO4)2transforms from Scheelite into Fergusonite structure accompanied with the volμmecollapse of10.03%. With the pressure increases to17.71GPa, the NaBi(WO4)2transforms again from Fergusonite into Monoclinic structure. With pressure higherthan28.6GPa, the NaBi(WO4)2turns into a amorphous state.2. The high-pressure studies on structural phase transition and the scatteringspectra have been performed on Ce: NaBi(WO4)2. Two pressure-induced structuralphase transitions (Scheelite Fergusonite Monoclinic) occurr at7.4GPa and19.34GPa accompanied with the volμme collapses of2.17%and18.3%, respectively. With pressure higher than32.5GPa, the Ce: NaBi(WO4)2turns into a amorphous state.3. The high-pressure studies on structural phase transition and the scatteringspectra have been performed on Nd: NaBi(WO4)2. Two pressure-induced structuralphase transitions (Scheelite Fergusonite Monoclinic) occurr at7.15GPa and20.83GPa accompanied with the volμme collapses of1.4%and14.2%, respectively. Withpressure higher than31GPa, the Ce: NaBi(WO4)2turns into a amorphous state.4. The high-pressure studies on structural phase transition and the scatteringspectra have been performed on NaBi(WO4)2doped Er and Yb. The twopressure-induced structural phase transitions (Scheelite Fergusonite Fergusonite)of Er: NaBi(WO4)2occurr at6.74GPa and18.88GPa accompanied with the volμmecollapses of1.6%and14.3%, respectively. The two pressure-induced structural phasetransitions (Scheelite Fergusonite Monoclinic) of Yb: NaBi(WO4)2occurr at6.4GPa and20.67GPa accompanied with the volμme collapses of1.3%and12.6%,respectively. With pressure higher than30.81GPa and30.44GPa, the NaBi(WO4)2doped Er and Yb turns into a amorphous state.5. Due to the disordering of the double tungstate, we could conclude from theexperimental result that the phase transition pressures and sequences of the system ofNaBi(WO4)2and its rare-earth doped compounds are not in conformity with thetungstate AWO4. Also, the phase transition pressures of the rare earth dopedNaBi(WO4)2decreases with the decreasing ionic radius of the doped rare-earth ions.All the system of NaBi(WO4)2and its rare-earth doped compounds transform into theamorphous state at high pressure, and these transformed pressure are all lower thanthe tungstate AWO4(40GPa). The results of this work provide important informationsto understand and sμmmarize the high-pressure structural phase transitions and thespectral properties of double tungstate. and its rare earth doped compounds.