Crystal structure of hexagonal RE(CO₃)OH

Abstract

Hexagonal rare earth carbonate hydroxides, RE(CO₃)OH, where RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er, were hydrothermally synthesized from formic acid and hydroxide gels of rare earth elements. The crystals exhibited bicephalous hexagonal prisms with lengths of several tens of micrometers. The crystal structures of a series of hexagonal RE(CO₃)OH were solved using the single crystal CCD-XRD intensity data sets. The space groups of the synthetic hexagonal RE(CO₃)OH crystals are all P-6. The present study has cast doubt upon the space group P-62c previously reported for the natural Ce(CO₃)OH, hydroxylbastnäsite-(Ce). The cell parameters decreased linearly with decreases in the ionic radii of the rare earth elements. La(CO₃)OH showed the largest unit cell (a = 12.6752(6), c = 10.0806(10) Å), while Er(CO₃)OH showed the smallest (a = 11.8977(4), c = 9.6978(8) Å). The rare earth atoms are in ninefold coordination with oxygen atoms to form a tricapped trigonal prism. The structure consists of layers of ²_∞[(OH)RE_3/3]²⁺ ions linked by carbonate ions. Raman spectra indicate the presence of carbonate and hydroxide groups. An evolutionary shift was observed from La to Er towards higher frequency, which was associated with a decreasing RE—O bond length.