Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb2As2O7, Pb(H2AsO4)2, Pb5(AsO4)3OH and NaPb4(AsO4)3 from Single-Crystal X-ray Data

Matthias Weil

doi:10.3390/min11111156

Abstract

Single-crystals of lead(II) oxoarsenates(V) were grown from the melt (Pb2As2O7), from solution (Pb(H2AsO4)2 and Pb5(AsO4)3OH), and under hydrothermal conditions (NaPb4(AsO4)3). Crystal structure refinements from single-crystal X-ray diffraction data revealed isotypism for both Pb2As2O7 and Pb(H2AsO4)2 with the corresponding barium and phosphate phases. A quantitative comparison of the crystal structures showed a high similarity for the isotypic M2X2O7 structures (M = Pb, Ba; X = As, P), whereas for the M(H2XO4)2 structures only the pair Pb(H2AsO4)2 and Pb(H2PO4)2 is similar, but not Ba(H2AsO4)2. Pb5(AsO4)3OH adopts the apatite structure type in space group P63/m, with the hydroxyl group disordered around Wyckoff position 2 b (0, 0, 0) in the channels of the structure. NaPb4(AsO4)3 represents a lacunar apatite with two of the three metal positions occupationally disordered by Pb and Na. In contrast to a previous X-ray powder study of NaPb4(AsO4)3 that reported an apatite-type structure in space group P63/m, the current single-crystal data clearly revealed a symmetry reduction to space group P3¯. Hence, NaPb4(AsO4)3 is the first lacunar apatite that comprises only tetrahedral anions and adopts the belovite structure type.

Highlights

Pb 2 AsV 2 O7, PbII (H2 AsV O4 )2, PbII 5 (AsV O4 )3 OH, and NaPbII 4 (AsV O4 )3 are reported and the results comparatively discussed with related phases, expanding our knowledge about the crystal chemistry of lead(II) oxoarsenates(V)
Single-crystals of lead(II) diarsenate(V) were obtained from the melt, starting with polycrystalline PbHAsO4. The latter was prepared by slow addition of aqueous solutions of Na2 HAsO4 (1.9 g in 50 mL water) to Pb(NO3 )2 (3.3 g in 50 mL water) in a molar ratio of
An amount of 0.5 g of PbHAsO4 was placed in a platinum crucible covered with a lid

Summary

Introduction

In the system Pb/As/O/(H), two approved and crystallographically fully characterized mineral phases are known to exist, viz. paulmooreite, PbII 2 AsV 2 O5 , and schultenite, PbII HAsV O4. Oxoarsenates(V) have been reported to exist and compiled in one of the older handbooks on lead and its compounds [8], structural details of these phases still are missing. Since these compounds might be present in nature as yet-unidentified mineral phases, detailed structure analyses were desirable. Pb 2 AsV 2 O7 , PbII (H2 AsV O4 ) , PbII 5 (AsV O4 ) OH, and NaPbII 4 (AsV O4 ) are reported and the results comparatively discussed with related phases, expanding our knowledge about the crystal chemistry of lead(II) oxoarsenates(V).

Syntheses and Single-Crystal Growth Procedures

Single-Crystal X-ray Diffraction and Structure Analysis

Results and Discussion

PbII 2 AsV 2 O7

Pb4 As2 O9