Neutron investigation of incommensurability and metastability in NH4HSeO

Abstract

This paper reports results of a neutron diffraction study on ${\mathrm{NH}}_{4}$${\mathrm{HSeO}}_{4}$ and ${\mathrm{ND}}_{4}$${\mathrm{DSeO}}_{4}$ single crystals. In ${\mathrm{NH}}_{4}$${\mathrm{HSeO}}_{4}$ the measurements give direct evidence for an incommensurate phase existing between the high-temperature monoclinic phase (space group B2) and the low-temperature ferroelectric lock-in phase (k=(1/3) (space group P1). Some of the high-temperature monoclinic, incommensurate, and ferroelectric phases are long-living metastable states. Room-temperature ${\mathrm{ND}}_{4}$${\mathrm{DSeO}}_{4}$, being orthorhombic (space group P${2}_{1}$${2}_{1}$${2}_{1}$), transforms at high temperatures into a stable incommensurate phase with the parent lattice of B2 symmetry. At even higher temperatures the monoclinic phase (space group B2) is restored. With decreasing temperature first the stable incommensurate phase, next a partly stable and partly metastable lock-in k=(1/4 phase, and then a metastable long-living ferroelectric k=(1/3 phase appears. At low temperatures, the orthorhombic phase P${2}_{1}$${2}_{1}$${2}_{1}$ is stable. Evidence for the growth processes of the P${2}_{1}$${2}_{1}$${2}_{1}$ phase within the lock-in k=(1/4 phase is given. Between the lock-in phases k=(1/4 and (1/3, states with several coexisting modulations have been observed on cooling. A group-theoretical analysis of the incommensurate phases is also made.