High-pressure experimental study on Rb₂S: antifluorite to Ni₂In-type phase transitions

The high-pressure behaviour of dirubidium sulfide, Rb₂S, with antifluorite-type structure under room conditions (space group ) has been studied up to 8 GPa at room temperature using angle-dispersive X-ray powder diffraction in a diamond-anvil cell (DAC). X-ray measurements have allowed us to completely characterize two phase transitions upon compression: (i) to an anticotunnite-type structure (Pnma) at some pressure between 1 bar and 0.7 GPa, and (ii) to a Ni₂In-type structure (P6₃/mmc) at 2.6 GPa. A gradual transition from the Pnma to the P6₃/mmc structures seems to occur between 2.6 and 4.5 GPa. These results are in excellent agreement with previous theoretical predictions. Strong luminescence is observed above 2.6 GPa (band maximum at 703 nm) when the transition to the Ni₂In-type phase starts to occur, the band maximum showing a non-linear blue shift with pressure. The observed sequence of phase transitions in Rb₂S is discussed in relation to the high-pressure structural behaviour of isomorphic sulfides and the structures are compared with the cationic arrays of their corresponding oxides (e.g. rubidium sulfate, in which the sulfide has been oxidized).

Computing details

For both compounds, program(s) used to refine structure: FULLPROF.

Figures

(phaseIII)

Crystal data

Rb2S	Z = 2
Mr = 203.00	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/mmc	µ = 29.79 mm^−1
Hall symbol: -P 6c 2c	T = 293 K
a = 5.30756 (8) Å	white
c = 6.80615 (4) Å	cylinder, 0.2 × 0.05 mm
V = 166.04 (1) Å^3	Specimen preparation: Prepared at 293 K and 5100000 kPa

Data collection

Xcalibur Oxford Difraction diffractometer	Data collection mode: transmission
Specimen mounting: diamond anvil cell	Scan method: Stationary detector

Refinement

Rp = 0.082	Profile function: pseudo-Voigt
Rwp = 0.193	8 parameters
Rexp = 0.085	0 restraints
χ^2 = 5.219
547 data points	Background function: 17 background points

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Rb1	0.00000	0.00000	0.00000	0.127 (11)*
Rb2	0.33333	0.66667	0.75000	0.102 (13)*
S1	0.33333	0.66667	0.25000	0.193 (8)*

(phaseII)

Crystal data

Rb2S	Z = 4
Mr = 203.00	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	µ = 23.85 mm^−1
Hall symbol: -P 2ac 2n	T = 293 K
a = 8.167 (10) Å	white
b = 5.101 (6) Å	cylinder, 0.2 × 0.05 mm
c = 9.957 (11) Å	Specimen preparation: Prepared at 293 K and 700000 kPa
V = 414.7 (8) Å^3

Data collection

Xcalibur Oxford Difraction diffractometer	Data collection mode: transmission
Specimen mounting: Diamond anvil cell	Scan method: Stationary detector

Refinement

Rp = 0.091	Profile function: pseudo-Voigt
Rwp = 0.159	14 parameters
Rexp = 0.073	0 restraints
χ^2 = 4.685
580 data points	Background function: 17 background points

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Rb1	0.0390 (10)	0.25000	0.1891 (4)	0.00000*
Rb2	0.1652 (6)	0.25000	0.5778 (5)	0.00000*
S1	0.224 (3)	0.25000	0.8892 (12)	0.00000*