Temperature-dependent diffraction studies on the phase evolution of tetraindium heptabromide

Abstract

The mixed-valent compound In₄Br₇ undergoes a higher-order phase transition below $\text{250}\text{K}$ which leads to a decrease in symmetry from the trigonal $\text{(R}\text{3}\text{̄}\text{m)}$ to the monoclinic (C2/c) system via $\text{R}\text{3}\text{̄}\text{c}$. The phase transition has been monitored by X-ray powder diffraction using a linear position-sensitive detector between 15 and $\text{300}\text{K}$, and the crystal structures at room temperature and at 90 K have been refined by means of time-of-flight neutron powder-diffraction data; at $\text{90}\text{K}$, the lattice parameters are $\text{a=13.066(1)}\text{A}\text{̊}$, $\text{b=7.520(1)}\text{A}\text{̊}$, $\text{c=31.105(1)}\text{A}\text{̊}$, and β=98.20(1)°; the new unit cell contains 88 atoms (Z=8) of which 12 are symmetry-independent. Due to their electronic instability because of a second-order Jahn–Teller effect, two of the three crystallographically independent monovalent indium cations are severely affected by the phase transition with respect to their coordination spheres; bond-valence calculations reveal significant strengthening of In⁺–Br⁻ bonding upon symmetry reduction. Structural changes and group–subgroup relationships as well as possible intermediate phases are discussed.

Introduction

Within the binary system In/Br, no fewer than seven compounds have been structurally characterized up to now, with In:Br stoichiometric ratios ranging from one (InBr) [1] to three (InBr₃) [2]; for an exhaustive compilation, see Ref. [3]. In these compounds, indium adopts the oxidation states one, two, and three. While trivalent indium is exclusively four- or six-coordinated by bromine anions, monovalent indium exhibits an astonishing variety of differing coordinations, with coordination numbers ranging from 7 to 12. The divalent state, on the other hand, occurs in the almost ethane-like dimeric unit (In₂^IIBr₆)²⁻ [1], [2], [4].

When it comes to chemical and physical properties, it is first worthwhile mentioning that low-valent indium bromides are extremely sensitive to oxidation and hydrolysis; furthermore, the title compound In₄Br₇ is sensitive to light and mechanical stress. A case of polymorphism in a low-valent indium bromide has been previously reported for In₅Br₇ by Ruck and Bärnighausen [5]. All these phenomena are strongly related with the exceptionally weak In⁺–Br⁻ bonding and the soft interatomic potential [6], [7] arising from a second-order Jahn–Teller instability due to the doubly-filled 5s atomic orbital on In⁺. As a structural consequence of this electronic situation, crystallographic studies have often found exceptionally large values for the In⁺ atomic displacement parameters together with empirical bond-valence sums well below one.

The crystal structure of In₄Br₇ is rich in complexity given the simple composition. It was first characterized at room temperature not too long ago (1995) on the basis of medium-resolved X-ray powder data, and it contains five crystallographically independent indium cations plus three bromine anions [8]. Thus, the crystal chemical formula is (In³⁺)₃(In⁺)₅(Br⁻)₁₄ and the trigonal unit cell (hexagonal setting) contains six formula units. A look into the crystal structure is given in Fig. 1.

Here, the indium cations are all stacked on top of each other along the hexagonal c-axis. While trivalent indium cations are either tetrahedrally or octahedrally coordinated by bromides with In³⁺–Br⁻ distances at $\text{2.47}\text{A}\text{̊}$ (tetrahedron) and $\text{2.67}\text{A}\text{̊}$ (octahedron), the monovalent indium cations exhibit coordination numbers of either 10—In(3)—or 12—In(4) and In(5)—with a wide In⁺–Br⁻ distance spectrum between ca. 3.3 and $\text{4.4}\text{A}\text{̊}$.

Already in 1995, a preliminary difference Fourier synthesis based on data taken from an In₄Br₇ crystal cooled to $\text{90}\text{K}$ suggested the possibility of a phase transition at low temperatures, simply due to the second-order Jahn–Teller instability of the monovalent indium atoms, most notably In(5) [8]. This prediction has recently been supported by Yamada and coworkers [9] who investigated several low-valent indium bromides by means of ${}^{115}\text{In-NMR}$, ${}^{115}\text{In-NQR}$, ${}^{81}\text{Br-NQR}$, and also by differential thermal analysis at temperatures between 77 and $\text{370}\text{K}$. In addition, the authors reported the trigonal phase of In₄Br₇ as being only metastable at ambient temperature, changing to an orthorhombic phase over a period of several weeks but reforming upon heating to $\text{420}\text{K}$. This finding is consistent with our own observation of single crystals of In₄Br₇ decomposing within weeks at room temperature even under argon atmosphere and in complete darkness.

Besides the phase instability of In₄Br₇ mentioned above, two NMR temperature effects were observed at 195 and $\text{95}\text{K}$. The point-group symmetry of the two different coordination polyhedra around In³⁺ as inferred from ${}^{81}\text{Br-NQR}$ is in favor of trigonal site symmetry down to $\text{77}\text{K}$. Recent experimental studies of ours [10], [11] on the phase behavior and crystal structure of In₄Br₇ let us now compare our results with the recent, independent nuclear-resonance studies by the Japanese group [9].