BeP(2)N(4) was synthesized in a multi-anvil apparatus starting from Be(3)N(2) and P(3)N(5) at 5 GPa and 1500 degrees C. The compound crystallizes in the phenakite structure type (space group R3, no. 148) with a=1269.45(2) pm, c=834.86(2) pm, V=1165.13(4) x 10(6) pm(3) and Z=18. As isostructural and isovalence-electronic alpha-Si(3)N(4) transforms into beta-Si(3)N(4) at high pressure and temperature, we studied the phase transition of BeP(2)N(4) into the spinel structure type by using density functional theory calculations. The predicted transition pressure of 24 GPa is within the reach of today's state of the art high-pressure experimental setups. Calculations of inverse spinel-type BeP(2)N(4) revealed this polymorph to be always higher in enthalpy than either phenakite-type or spinel-type BeP(2)N(4). The predicted bulk modulus of spinel-type BeP(2)N(4) is in the range of corundum and gamma-Si(3)N(4) and about 40 GPa higher than that of phenakite-type BeP(2)N(4). This finding implies an increase in hardness in analogy to that occurring for the beta- to gamma-Si(3)N(4) transition. In hypothetical spinel-type BeP(2)N(4) the coordination number of phosphorus is increased from 4 to 6. So far only coordination numbers up to 5 have been experimentally realized (gamma-P(3)N(5)), though a sixfold coordination for P has been predicted for hypothetic delta-P(3)N(5). We believe, our findings provide a strong incentive for further high-pressure experiments in the quest for novel hard materials with yet unprecedented structural motives.

HP-Ca2Si5N8 was obtained by means of high-pressure high-temperature synthesis utilizing the multianvil technique (6 to 12 GPa, 900 to 1200 degrees C) starting from the ambient-pressure phase Ca2Si5N8. HP-Ca2Si5N8 crystallizes in the orthorhombic crystal system (Pbca (no. 61), a=1058.4(2), b=965.2(2), c=1366.3(3) pm, V=1395.7(7)x10(6) pm3, Z=8, R1=0.1191). The HP-Ca2Si5N8 structure is built up by a three-dimensional, highly condensed nitridosilicate framework with N[2] as well as N[3] bridging. Corrugated layers of corner-sharing SiN4 tetrahedra are interconnected by further SiN4 units. The Ca2+ ions are situated between these layers with coordination numbers 6+1 and 7+1, respectively. HP-Ca2Si5N8 as well as hypothetical orthorhombic o-Ca2Si5N8 (isostructural to the ambient-pressure modifications of Sr2Si5N8 and Ba2Si5N8) were studied as high-pressure phases of Ca2Si5N8 up to 100 GPa by using density functional calculations. The transition pressure into HP-Ca2Si5N8 was calculated to 1.7 GPa, whereas o-Ca2Si5N8 will not be adopted as a high-pressure phase. Two different decomposition pathways of Ca2Si5N8 (into Ca3N2 and Si3N4 or into CaSiN2 and Si3N4) and their pressure dependence were examined. It was found that a pressure-induced decomposition of Ca2Si5N8 into CaSiN2 and Si3N4 is preferred and that Ca2Si5N8 is no longer thermodynamically stable under pressures exceeding 15 GPa. Luminescence investigations (excitation at 365 nm) of HP-Ca2Si5N8:Eu2+ reveal a broadband emission peaking at 627 nm (FWHM=97 nm), similar to the ambient-pressure phase Ca2Si5N8:Eu2+.

The high‐pressure behavior of group II element nitrides M3N2 (M=Be, Mg, Sr, and Ba) was studied up to 100 GPa and beyond. Evaluating a manifold of hypothetical polymorphs of composition A3X2 leads to proposing a new high‐pressure polymorph of Be3N2 with an anti‐A‐sesquioxide structure appearing at 82 GPa. Two high‐pressure phases were found for Mg3N2: first an anti‐B‐sesquioxide‐type structure should appear at 21 GPa followed by an anti‐A‐sesquioxide‐type structure at 65 GPa. While structure and true nature of Sr3N2 and Ba3N2 are not yet experimentally determined, we identified an anti‐bixbyite structure to be the ground state structure of Sr3N2 and a distortion variant of the anti‐A‐sesquioxide‐type structure as lowest energy modification for Ba3N2. For Sr3N2 the sequence of high‐pressure phases are (1) an anti‐Rh2O3‐II structure appearing at 3 GPa, (2) an anti‐B‐sesquioxide structure becoming most stable at 12 GPa and (3) a hexagonal P 63/mmc structure favored at 26 GPa. The development of the c /a ‐ratio of anti‐A‐sesquioxide Ba3N2 under pressure was examined, revealing a gradual reduction under pressure. Three high‐pressure polymorphs are then further proposed for Ba3N2: (1) an anti‐Rh2O3‐II structure at 2 GPa, (2) an anti‐CaIrO3 structure at 32 GPa and (3) a hexagonal P 63/mmc structure at 52 GPa. The results for all group II element nitrides M3N2 were compared. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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The high-pressure behavior of Ca 3 N 2 is studied up 100 GPa using density functional theory. Evaluation of many hypothetical polymorphs of composition A 3 X 2 leads us to propose four high-pressure polymorphs for both R-and -Ca 3 N 2 : (1) an anti-Rh 2 O 3 -II structure at 5 GPa, (2) an anti-B-sesquioxide structure at 10 GPa, (3) an anti-A-sesquioxide structure at 27 GPa, and (4) a hitherto unknown hexagonal structure (P6 3 /mmc), derived from the post-perovskite structure of CaIrO 3 , at 38 GPa. The development of the density and bulk modulus under pressure has been examined.