The new cesium pentaborate HP-CsB5 O8 is synthesized under high-pressure/high-temperature conditions of 6 GPa and 900 °C in a Walker-type multianvil apparatus. The compound crystallizes in the orthorhombic space group Pnma (Z=4) with the parameters a=789.7(1), b=961.2(1), c=836.3(1) pm, V=0.6348(1) nm(3) , R1 =0.0359 and wR2 =0.0440 (all data). The new structure type of HP-CsB5 O8 exhibits the simultaneous linkage of trigonal BO3 groups, corner-sharing BO4 tetrahedra, and edge-sharing BO4 tetrahedra including the presence of threefold-coordinated oxygen atoms. With respect to the rich structural chemistry of borates, HP-CsB5 O8 is the second structure type possessing this outstanding combination of the main structural units of borates in one compound. The structure consists of corrugated chains of corner- and edge-sharing BO4 tetrahedra interconnected through BO3 groups forming octagonal channels. Inside these channels, cesium is 13+3-fold coordinated by oxygen atoms. (11) B MQMAS NMR spectra are analyzed to estimate the isotropic chemical shift values and quadrupolar parameters. IR and Raman spectra are obtained and compared to the calculated vibrational frequencies at the Γ-point. The high-temperature behavior is examined by means of temperature-programmed powder diffraction.

The rubidium triborate HP-RbB 3 O 5 (HP = high-pressure) was synthesized under highpressure/high-temperature conditions of 6 GPa and 1000 • C in a Walker-type multianvil apparatus. The precursor was gained from a mixture of rubidium carbonate Rb 2 CO 3 and boric acid H 3 BO 3 heated at 850 • C under normal pressure conditions. The single-crystal structure determination showed that HP-RbB 3 O 5 is isotypic to HP-KB 3 O 5 , crystallizing monoclinically with eight formula units in the space group C2/c possessing the lattice parameters a = 982.3(2), b = 885.9(2), c = 919.9(2) pm, and β = 104.0(1) • . The boron-oxygen framework consists of trigonal-planar BO 3 groups as well as corner-and edge-sharing BO 4 tetrahedra that are connected to a three-dimensional framework. Therein, the rubidium cations are surrounded by 10 oxygen anions. IR-and Raman-spectroscopic investigations were performed on single crystals of the compound.

The new high-pressure borate HP-Cs1-x (H3 O)x B3 O5 (x=0.5-0.7) was synthesized under high-pressure/high-temperature conditions of 6 GPa/900 °C in a Walker-type multianvil apparatus. The compound crystallizes in the monoclinic space group C2/c (Z=8) with the parameters a=1000.6(2), b=887.8(2), c=926.3(2) pm, β=103.1(1)°, V=0.8016(3) nm(3) , R1=0.0452, and wR2=0.0721 (all data). The boron-oxygen network is analogous to those of the compounds HP-MB3 O5 , (M=K, Rb) and exhibits all three structural motifs of borates-BO3 groups, corner-sharing BO4 tetrahedra, and edge-sharing BO4 tetrahedra-at the same time. Channels inside the boron-oxygen framework contain the cesium and oxonium ions, which are disordered on a specific site. Estimating the amount of hydrogen by solid-state NMR spectroscopy and X-ray diffraction led to the composition HP-Cs1-x (H3 O)x B3 O5 (x=0.5-0.7), which implies a nonzero phase width.

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The new polymorph of sodium tetraborate HP-Na 2 B 4 O 7 was synthesized under high-pressure / high-temperature conditions of 6 GPa and 1000°C in a multianvil apparatus with a Walker-type module. HP-Na 2 B 4 O 7 crystallizes with nine formula units per cell in the trigonal chiral space groups P3 2 21 or P3 1 21. The parameters are a = * Univ.-Prof. Dr. H. Huppertz Fax: +43 (0)512 507 2934 E-Mail: Hubert.Huppertz@uibk.ac.at [a] 81 765.5(2), c = 2142.3(4) pm, V = 1.0872(3) nm 3 , R 1 = 0.0581, and wR 2 = 0.0809 (all data). The crystal structure of HP-Na 2 B 4 O 7 is built up from interconnected "sechser" rings of alternating corner-sharing BO 3 and BO 4 groups.