High-pressure polymorph of LaNbO4

A new polymorph of LaNbO4 with a BaMnF4-type structure (a = 0.3938(2) nm, b = 1.442(1) nm, c = 0.5682(3) nm, Z = 4) and octahedral oxygen coordination of Nb was prepared at high pressures and temperatures (8 GPa, 1570 K). At normal pressure, the reverse transformation, from high-pressure LaNbO4 to t he M-fergusonite form, occurs at temperatures above 870 K and is close to a second-order phase transition. The spontaneous polarization in high-pressu re LaNbO4, evaluated from the second-order nonlinear optical response, is a bout 2 mu C/cm(2).