Equation of State and Amorphization of Ca₉R(VO₄)₇ (R = La, Nd, Gd): A Combined High-Pressure X-ray Diffraction and Raman Spectroscopy Study

Abstract

Ca9R­(VO4)7 (R = rare earth) multicomponent oxides of a whitlockite-related structure are under consideration for applications in optoelectronics. In this work, the Czochralski-grown Ca9R­(VO4)7 crystals were investigated as a function of pressure by powder X-ray diffraction and single-crystal Raman spectroscopy. The diffraction experiments were performed at the ALBA synchrotron under pressures ranging up to 9.22(5), 10.7(1), and 8.55(5) GPa for R = La, Nd, and Gd, respectively, to determine the third order equation of state (EOS) parameters. Fitting of the Birch–Murnaghan EOS provided the isothermal bulk moduli K 0 = 63(4), 63(2), and 61(5) GPa for these three orthovanadates. These values are apparently lower than that reported for structurally related tricalcium vanadate Ca3(VO4)2. The compressibility anisotropy was observed; the lattice is markedly stiffer in [001] than in [100] direction. For Ca9Nd­(VO4)7, the variation of the diffractograms just above 10 GPa provides an indication on the beginning of amorphization process; during pressure release the whitlockite-like structure is recovered. Raman spectroscopy measurements for single crystals of the above-mentioned rare-earth vanadates and Ca9Y­(VO4)7 were performed (the maximum pressures achieved were 16.3(1), 21.2(1), 15.3(1), and 18.6(1) GPa for R = Y, La, Nd, and Gd, respectively). These measurements reveal a partially reversible phase transition interpreted as amorphization, with an onset at the pressure of ∼9–10 GPa, characterized by broadening of the peaks and their shift to lower energies.

The pressure and temperature evolution of the Ca3V2O8 crystal structure using powder X-ray diffraction

“…As discussed in the next paragraph, the bulk modulus at zero pressure ( B 0 ) and its pressure derivative () agree within confidence ellipses with the values obtained from the second-order EOS. The B 0 obtained is close to the bulk moduli found for other calcium compounds with the whitlockite-related structure, such as Ca 9 R(VO 4 ) 7 (R = La, Nd, Gd) 19 and β-Ca 3 P 2 O 8. 67,68 The reported bulk modulus for the low-pressure trigonal Ca 3 V 2 O 8 phase is much smaller that of other orthovanadates with similar chemical formulae, for example: Mn 3 V 2 O 8 , Co 3 V 2 O 8 , Ni 3 V 2 O 8 , and Zn 3 V 2 O 8 , however these compounds have different crystal structures which have been recently systematically studied.…”

“…18 One Raman study on the related compound Ca 9−x R(VO 4 ) 7 (R = La, Nd, Gd) reported signs of partial amorphization around 9-10 GPa. 19 According to the results summarized above, the HP behavior of Ca 3 V 2 O 8 is different from that of other orthovanadates with equivalent chemical formulae. For instance, Zn 3 V 2 O 8 , Ni 3 V 2 O 8 , and Ba 3 V 2 O 8 remain stable up to at least 15, 20 23 21 and 29 GPa 22 respectively.…”

High pressure crystal structures of orthovanadates and their properties

“…delafossite-type vanadates (e.g. LiVO2) [176], and metavanadates (e. g. Ca3(VO4)2 [177]). All these compounds have practical applications and his study under compression could lead to important discoveries.…”