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Abstract:
Unit-cell parameters of La3Nb0.5Ga5.5O14 (langanite) have been determined at a number of pressures up to 23 GPa in diamond anvil high pressure cells with single crystal X-ray diffraction techniques. The anisotropic behaviour of unit cell parameters of langanite is caused by differences of bonding strength in direction parallel to the a- and c-axis. Within the investigated pressure range, the c/a-ratio increases from 0.6232 to 0.6503. The volume compressibility of trigonal langanite (space group P321) is the 0.007 GPa(-1), with calculated bulk modulus of 145(3) GPa (B-0' = 1.4(8)). At a pressure of 12.4(3) GPa a first-order phase transition was detected, characterised by a discontinuity in the pressure dependence of lattice parameter c. The high-pressure phase (presumably monoclinic, A2) with a volume compressibility of 0.011 GPa(-1) (B-0 = 93(2) GPa, B-0' = 1.9(9)) is more compressible as compared to the initial phase due to an abnormal pressure dependence of lattice parameter a.
