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The behavior of alkaline carbonates at high pressure is poorly understood. Indeed, theoretical and experimental investigations of the pressure induced structural changes have appeared in the literature only sporadically. In this article we use evolutionary crystal structure prediction algorithms based on density functional theory to determine crystal structures of high-pressure phases of Li2CO3, Na2CO3, and K2CO3. Our calculations reveal several new structures for each compound in the pressure range of 0–100 GPa. Cation arrays of all high-pressure structures are of the AlB2 topological type. The comparison of cation arrays of ambient and high-pressure structures with that of binary A2B compounds indicates an analogy between high-pressure behavior of alkaline carbonates and alkaline sulfides (oxides, selenides, tellurides), which under compression go through the following series of phase transitions: anti-CaF2 → anti-PbCl2 → Ni2In → AlB2. All structures presented in this trend are realized in the high-pressure trend of alkaline carbonates, although some intermediary structures are omitted for particular compounds.

Crystals of Na2Ca­(CO3)2, the structural analogues of mineral nyerereite, were synthesized using hydrothermal technique at 1 kbar and 450 °C. The crystals are transformational twins formed at the transition from the high-temperature hexagonal modification to the low-temperature orthorhombic modification. The structure was solved and refined to R = 0.059 in P21 ca (No. 29) space group with a = 10.0713(5) Å, b = 8.7220(2) Å, and c = 12.2460(4) Å. The only structural analogue of the synthesized crystal is the high-temperature modification of K2Ca­(CO3)2, which can be considered as a disordered analogue of Na2Ca­(CO3)2. Structural analogues among borates and other classes of compounds have not been found. Based on group–subgroup analysis, we propose the structures of high- and intermediate-temperature modifications of Na2Ca­(CO3)2. The relations of the determined structure with other polymorphs of Na2Ca­(CO3)2 have also been considered.

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A comprehensive study of the BaF2–Ba3(BO3)2 phase diagram has revealed a significant difference between the two intermediate phases Ba5(BO3)3F and Ba7(BO3)4−yF2+3y. The latter exhibited (BO3)3−↔ 3F− anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4−yF2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3−↔ 3F− isomorphism, allows for the disclosure of the mechanism of (BO3)3−↔ 3F− heterovalent anionic substitution in fluoride borates via [(BO3)F]4− tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO3)3−↔ 3F− substitution.

Single crystals of the new double alkali carbonate Ca 3 Na 2 (CO 3 ) 4 were synthesized using a high-pressure large volume apparatus at 6 GPa by slow cooling of the stoichiometric carbonate mixture from 1400 to 1150 °C for 2.5 h. The structure was solved and refined to R = 0.044 using 7489 independent reflections: P1n1, Z = 8, a = 31.4421(8) Å, b = 8.1960(2)Å, c = 7.4360(2) Å, and β = 89.923(2)°. The structure is characterized by the maximal number of nonparallel sets of CO 3 groups among carbonates. The compound is homeotypic with the orthoborates M 3 Ln 2 (BO 3 ) 4 (where M = Ca, Sr, Ba and Ln = Er, Sm, Nd, Pr, La, Y, Gd, Eu, Dy, Ho, and Bi). No homeotypic analogs have been found among carbonates. The structure is described as a packing of two-capped trigonal prisms formed by Na + and Ca 2+ cations and centered by CO 3 triangles.

Detailed study of the BaB 2 O 4 -BaF 2 -BaO system resulted in the discovery of the new Ba 7 (BO 3 ) 42x F 2+3x solid solution belonging to the BaF 2 -Ba 3 (BO 3 ) 2 section. The distinguishing feature of the crystal structure of Ba 7 (BO 3 ) 42x F 2+3x phase is its extensive (BO 3 ) 32 « 3F 2 anionic isomorphic substitution, confirmed by X-ray diffraction study of Ba 7 (BO 3 ) 3.51 F 3.47 (x = 0.49) single crystals (space group P6 3 ; a = 11.18241(11) A ˚, c = 7.23720(8) A ˚). The area of homogeneity for Ba 7 (BO 3 ) 42x F 2+3x solid solution spans between Ba 7 (BO 3 ) 3.35 F 3.95 and Ba 7 (BO 3 ) 3.79 F 2.63 compositions (0.21 , x , 0.65). Also, a new orthorhombic phase with a tentative composition of Ba 5 (BO 3 ) 3 F has been identified in XRD powder patterns and indexed with cell parameters a = 7.605 A ˚, b = 14.843 A ånd c = 10.291 A ˚.

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