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

Abstract

Abstract Transparent single crystals of the scheelite-type Ln[AsO4] 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, I41/a) for La[AsO4] (a = 516.92(4), c = 1186.1(9) pm), Ce[AsO4] (a = 514.60(1), c = 1175.44(2) pm), Pr[AsO4] (a = 512.63(4), c = 1168.25(9) pm), and Nd[AsO4] (a = 510.46(4), c = 1160.32(11) pm) as compared to the well-known monazite-type phases (monoclinic, P21/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 [AsO4]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 [LnO8+1]15– polyhedra with the [AsO4]3– tetrahedra in the monazite- resulting in exclusively vertex connected [LnO8]13– and [AsO4]3– units in the scheelite-type structure.