High-pressure investigations of lanthanoid oxoarsenates: I. Single crystals of scheelite-type <i>Ln</i>[AsO₄] phases with <i>Ln</i> = La–Nd from monazite-type precursors

Abstract

Abstract Transparent single crystals of the scheelite-type Ln [AsO 4 ] phases with Ln = La–Nd are obtained by the pressure-induced monazite - to-scheelite type phase transition in a Walker-type module under high-pressure and high-temperature conditions of 11 GPa at 1100–1300 °C. Coinciding with this transition, there is an increase in density and a reduction in molar volume of about 4.5 % for the scheelite-type phases (tetragonal, I 4 1 / a ) for La[AsO 4 ] ( a = 516.92(4), c = 1186.1(9) pm), Ce[AsO 4 ] ( a = 514.60(1), c = 1175.44(2) pm), Pr[AsO 4 ] ( a = 512.63(4), c = 1168.25(9) pm), and Nd[AsO 4 ] ( a = 510.46(4), c = 1160.32(11) pm) as compared to the well-known monazite-type phases (monoclinic, P 2 1 / n ). Surprisingly enough, the scheelite-type oxoarsenates(V) exhibit a lower coordination number for the Ln 3+ cations ( CN = 8 versus CN = 8 + 1), whereas the isolated tetrahedral [AsO 4 ] 3– anions ( d (As–O) = 168.9–169.3 pm for the scheelites as compared to d (As–O) = 167.1–169.9 pm for the monazites) remain almost unchanged. So the densification must occur because of the loss of two edge-connections of the involved [ Ln O 8+1 ] 15– polyhedra with the [AsO 4 ] 3– tetrahedra in the monazite- resulting in exclusively vertex connected [ Ln O 8 ] 13– and [AsO 4 ] 3– units in the scheelite-type structure.