The Zintl phase CaSi2 commonly occurs in the 6R structure where puckered hexagon layers of Si atoms are stacked in an AA'BB'CC' fashion. In this study we show that sintering of CaSi2 in a hydrogen atmosphere (30 bar) at temperatures between 200 and 700 degrees C transforms 6R-CaSi2 quantitatively into 3R-CaSi2. In the 3R polymorph (space group R-3m (no. 166), a=3.8284(1), c=15.8966(4), Z=3) puckered hexagon layers are stacked in an ABC fashion. The volume per formula unit is about 3% larger compared to 6R-CaSi2. First principles density functional calculations reveal that 6R and 3R-CaSi2 are energetically degenerate at zero Kelvin. With increasing temperature 6R-CaSi2 stabilizes over 3R because of its higher entropy. This suggests that 3R-CaSi2 should revert to 6R at elevated temperatures, which however is not observed up to 800 degrees C. 3R-CaSi2 may be stabilized by small amounts of incorporated hydrogen and/or defects.

Hydrides GdGaH were obtained by exposing the Zintl phase GdGa with the CrB structure to a hydrogen atmosphere at pressures from 1.5 to 50 bar and temperatures from 50 to 500 degrees C. Structural analysis by powder X-ray diffraction suggests that conditions with hydrogen pressures in a range between 15 and 50 bar and temperatures below 500 degrees C afford a uniform hydride phase with the NdGaH1.66 structure (Cmcm, a=3.9867(7) angstrom, b=12.024(2) angstrom, c=4.1009(6) angstrom) which hosts H in two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Gd atoms, whereas H2 atoms are inserted between Ga atoms. The assignment of the NdGaH1.66 structure is corroborated by first principles DFT calculations. Modeling of phase and structure stability as a function of composition resulted in excellent agreement with experimental lattice parameters when x=1.66 and revealed the presence of five-atom moieties Ga-H2-Ga-H2-Ga in GdGaH1.66. From in situ powder X-ray diffraction using synchrotron radiation it was established that hydrogenation at temperatures above 200 degrees C affords a hydride with x approximate to 1.3, which is stable up to 500 degrees C, and that additional H absorption, yielding GdGaH1.66, takes place at lower temperatures. Consequently, GdGaH1.66 desorbs H above T=200 degrees C. Without the presence of hydrogen, hydrides GdGaHx decompose at temperatures above 300 degrees C into GdH2 and an unidentified Gd-Ga intermetallics. Thus the hydrogenation of GdGa is not reversible. From magnetic measurements the Curie Weiss constant and effective magnetic moment of GdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of similar to 8 mu B which is typical for compounds containing Gd3+ ions.

Hydrides Eu₃Si₄H_2+x were obtained by exposing the Zintl phase Eu₃Si₄ to a hydrogen atmosphere at a pressure of 30 bar and temperatures from 25 to 300 °C. Structural analysis using powder X-ray diffraction (PXRD) data suggested that hydrogenations in a temperature range 25 – 200 ºC afford a uniform hydride phase with an orthorhombic structure (Immm, a ≈ 4.40 Å, b ≈ 3.97 Å, c ≈ 19.8 Å), whereas at 300 ºC mixtures of two orthorhombic phases with c ≈ 19.86 and ≈19.58 Å were obtained. The assignment of a composition Eu₃Si₄H_2+x is based on first principles DFT calculations, which indicated a distinct crystallographic site for H to be occupied in the Eu₃Si₄ structure. In this position, H atoms are coordinated in a tetrahedral fashion by Eu atoms. The resulting hydride Eu₃Si₄H₂ is stable by -0.46 eV/H atom with respect to Eu₃Si₄ and gaseous H₂. Deviations between the lattice parameters of the DFT optimized Eu₃Si₄H₂ structure and the ones extracted from PXRD patterns point to the presence of additional H in interstitials also involving Si atoms. Subsequent DFT modeling of compositions Eu₃Si₄H₃ and Eu₃Si₄H₄ showed considerably better agreement to the experimental unit cell volumes. However, the ordered monoclinic model structures do not provide a good match to the experimental, orthorhombic, PXRD patterns. It was then concluded that the hydrides of Eu₃Si₄ have a composition Eu₃Si₄H_2+x (x < 2) and are disordered with respect to H in Si₂Eu₃ interstitials. Hydrides Eu₃Si₄H_2+x decompose at temperatures above 300 °C in a dynamic vacuum into unidentified products. Thus the hydrogenation of Eu₃Si₄H_2+x is not reversible. From magnetic measurements the Curie-Weiss constant and effective magnetic moment of Eu₃Si₄H_2+x were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of ~8 m_B which is typical for compounds containing Eu²⁺ 4f⁷ ions.  

The β–α (order–disorder) transition in the silanides ASiH₃ (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (C_p), solid-state ²H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH₃ corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH₃^– moieties. At temperatures below 200 K ASiH₃ exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH₃ and KSiH₃, respectively. The transition is accompanied by a large molar volume increase of about 14%. The C_p(T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH₃^– rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH₃, the SiH₃^– anions undergo rotational diffusion with average relaxation times of 0.2–0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20–40 K). ²H NMR revealed that the α-form can coexist, presumably as 2–4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from C_p measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH₃ near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH₃^– ions are only weakly coordinated in an environment of A⁺ cations. The orientational flexibility of SiH₃^– can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations.

We investigated the hydride reduction of tetragonal BaTiO₃ using the metal hydrides CaH₂, NaH, MgH₂, NaBH₄ and NaAlH₄. The reactions employed molar BaTiO₃:H ratios of up to 1.8 and temperatures near 600 °C. The air stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA) and solid-state ¹H NMR spectroscopy. PXRD showed the formation of cubic products - indicative of the formation of BaTiO_3-xH_x - except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH₄ reduces samples) and 4.033 Å (for MgH₂ reduced samples). With increasing BaTiO₃:H ratio, CaH₂, NaAlH₄ and MgH₂ reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increase of up to 3.5 % for reduced BaTiO₃, suggesting that metal hydride reduction yielded oxyhydrides BaTiO_3-xH_x with x values larger 0.5. ¹H NMR, however, revealed rather low concentrations of H, and, thus a simultaneous presence of O vacancies in reduced BaTiO₃. It has to be concluded that hydride reduction of BaTiO₃ yields complex disordered materials BaTiO_3-xH_y□_(x-y) with x up to 0.6, y in a range 0.05 – 0.2 and (x-y) > y, rather than homogeneous solid solutions BaTiO₃H_x. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of chemical shifts and breadth of the signals signifies the structural disorder in BaTiO_3-xH_y□_(x-y). Sintering of BaTiO_3-xH_y□_(x-y) in a gaseous H₂ atmosphere resulted in more ordered materials as indicated by considerably sharper ¹H resonances.