Phase coexistence and hysteresis effects in the pressure-temperature phase diagram of NH${}_3$BH${}_3$

The potential hydrogen storage compound NH${}_3$BH${}_3$ has three known structural phases in the temperature and pressure ranges 110–300 K and 0–1.5 GPa, respectively. We report here the boundaries between, and the ranges of stability of, these phases. The phase boundaries were located by in situ measurements of the thermal conductivity, while the actual structures in selected areas were identified by in situ Raman spectroscopy and x-ray diffraction. Below 0.6 GPa, reversible transitions involving only small hysteresis effects occur between the room-temperature tetragonal plastic crystal $I$4$\mathit{mm}$ phase and the low-temperature orthorhombic Pmn2${}_1$ phase. Transformations of the $I$4$\mathit{mm}$ phase into the high-pressure orthorhombic Cmc2${}_1$ phase, occurring above 0.8 GPa, are associated with very large hysteresis effects, such that the reverse transition may occur at up to 0.5 GPa lower pressures. Below 230 K, a fraction of the Cmc2${}_1$ phase is metastable to atmospheric pressure, suggesting the possibility that dense structural phases of NH${}_3$BH${}_3$, stable at room temperature, could possibly be created and stabilized by alloying or by other methods. Mixed orthorhombic Pmn2${}_1$/Cmc2${}_1$ phases were observed in an intermediate pressure-temperature range, but a fourth structural phase predicted by Filinchuk et al. [Phys. Rev. B 79, 214111 (2009)] was not observed in the pressure-temperature ranges of this experiment. The thermal conductivity of the plastic crystal $I$4$\mathit{mm}$ phase is about 0.6 W m${}^{-1}$ K${}^{-1}$ and only weakly dependent on temperature, while the ordered orthorhombic phases have higher thermal conductivities limited by phonon-phonon scattering.

Figure 1

Thermal conductivity as a function of temperature at the pressures (in GPa) indicated. The anomalous data set at 0.75 GPa is shown as a solid curve to make it easier for the reader to find(see text).

Figure 2

Thermal conductivity data from three pressure cycles at room temperature. Note the very large hysteresis of the $I$4$\mathit{mm}$-Cmc2${}_1$ transformation.

Figure 3

Measured data for the specific heat per unit volume as a function of temperature at 0.1, 0.3, and 0.6 GPa. The small rise below 150 K is an artifact of the method used.

Figure 4

Thermal conductivity as a function of pressure, measured during two pressure cycles near 245 K. Transformations between the low-pressure tetragonal $I$4$\mathit{mm}$ phase and the orthorhombic Pmn2${}_1$ phase are clearly seen (see text for details).

Figure 5

Pressure-temperature phase diagram of NH${}_3$BH${}_3$ derived in the present paper. Symbols show coordinates of the transformations observed, and arrows show both the transformation direction and the approximate transition ranges observed. Full (green) line denotes the $I$4$\mathit{mm}$↔Pmn2${}_1$ transition, dashed lines transformations between $I$4$\mathit{mm}$ and Cmc2${}_1$ phases. Dotted, almost-vertical line denotes approximate locations for the Pmn2${}_1$→Cmc2${}_1$ transformation.

Figure 6

Thermal conductivity as a function of temperature, measured during heating at atmospheric pressure after producing the orthorhombic Cmc2${}_1$ phase at 1 GPa at room temperature.

Figure 7

Raman spectra recorded from NH${}_3$BH${}_3$ upon cooling at 1 GPa.

Figure 8

Comparison between Raman spectra of the known phases of NH${}_3$BH${}_3$ and the mixed phase obtained below 280 K at intermediate pressures.

Figure 9

XRD patterns recorded from NH${}_3$BH${}_3$ upon cooling at 0.8 GPa. Indexing for some reflections from the Pmn2${}_1$ phase (p) is shown on the 245 K pattern and for the Cmc2${}_1$ phase on the 220 K pattern.

Figure 10

Rietveld refinement profile for the mixed orthorhombic phase (top) together with the contributions from the separate phases (bottom two curves). Tics corresponding to peak positions for the two phases (Cmc2${}_1$ top, Pmn2${}_1$ below) are shown together with the residual difference below the refinement profile (for fitted lattice parameters, see text).

Figure 11

Thermal conductivity data measured during three temperature cycles at 0.65 GPa. See text for details.