Crystal Chemistry of the adelite-descloizite type structure

The minerals and analogue synthetic compounds belonging to the adelite- and descloizite-group are hydrated arsenates, vanadates, silicates, and molybdates. There are two cation sites (atoms M1 = Na/Ca/Cd/Pb and M2 = Mg/Al/Mn/Fe/Co/Ni/Cu/Zn) exhibiting a distinct stereochemical behaviour: the first form large coordination polyhedra whereas the latter are in an octahedral [6] coordination. All compounds have orthorhombic symmetry. The atomic arrangements within the two series are closely related to each other. However, there is a centrosymmetric (preferred by lead-vanadates) and an acentric sub-type (preferred by calcium-arsenates/silicates) but the assignment is not quite clear. Despite of the variable symmetry the main difference between the two sub- types is a changing [7] or [8] coordination for the M1 site. It is worthy to note that within the adelite- and descloizite-group usually the larger Pb atoms have the smaller coordinaiton number as compared to Ca atoms. The electron configuration of divalent positively charged Ca atoms is characterized by filled electron shells whereas that of divalent positively charged Pb atoms exhibits a lone-electron pair. The aim of the present project is to investigate the reason for the formation of two structural sub-types. The synthesis of compounds substituting Sr atoms at the M1 site allows a comparison between compounds with well comparable ionic radii for all atoms (they are practically the same for divalent Sr and Pb atoms) but with different electron configuration. Further, the substitution by Ba atoms at the M1 site allows an additional variation of the ratios of ionic radii for the components forming this structure type. Besides the experimental part a theoretical approach will define the acentricity (it is not clear yet whether the deviation from centrosymmetry is the same for all acentric crystallizing compounds). From Bi-bearing minerals belonging to these structure families it is evident that divalent Ca/Pb cations may be substituted at least in parts by tri-valent Bi atoms. As a consequence the charge balance has to be achieved by a substitution of hydroxyl groups by oxo-oxygen atoms. Due to the fact that these substitution paths are of general interest in crystal- and stereo-chemistry a pure end-member exhibiting Bi atoms is aspired to be synthesized. Bi- Cu-vanadates are on interest for in connection with oxygen-ion-conducting ceramics and solid state batteries.

Arsenic has been classified at the top of the priority list of the most hazardous substances. Studies of the arsenate behaviour in soils, sediments and natural waters that have been subjected to pollution require a precise knowledge of the possible precipitating phases, their crystal chemistry, and their thermodynamic properties. The role of hydrogen atoms controlling variants of crystal structures have been overseen very often because of weak detection signals. The tsumcorite group consists of about 30 natural and synthetic compounds with the general formula M1M2 2 (XO 4 ) 2 (H2 O,OH)2 . For each of the cation sites M1, M2 and X at least two different valences are possible; the coupled exchange at these cation positions ensures electro-neutrality. Furthermore, charge is balanced by adjusting the ratio OH:H 2 O. Most members are monoclinic forming H3 O2 groups, i.e., a strong symmetry- restricted hydrogen bond between two OH groups linked by a centre of inversion (`tsumcorite type`). Two structure variants with lower (triclinic) symmetry are caused by distinct crystal-chemical requirements: In the `helmutwinklerite type` two H2 O molecules per formula unit require an avoidance of the symmetry-restriction of the hydrogen bond. Secondly, divalent copper ions besides trivalent iron ions necessitate a mechanism for individual environments at their M2 sites; the inversion centres are maintained (`gartrellite type`). Hydrothermal syntheses produced the new compound SrCo2 (AsO4 )(AsO3 OH)(OH)(H 2 O) which represents the first proof of (partially) protonated arsenate tetrahedra in this group of compounds: `Sr-Co type`. The As-atom site splits into two crystallographically independent sites. Monoclinic symmetry is maintained but the symmetry restriction of the hydrogen bond is given up: the former equivalent oxygen-atom sites split into one representing a H 2 O molecule whereas the other one is occupied by a hydroxyl group. The formation of protonated arsenate groups is controlled by the pH and Eh condition during syntheses. A further complex variation of a parental structure is verified in the triclinic MHXO4 compounds (CaHPO 4 , CaHAsO4 , a-NaHSO4 , a-SrHPO 4 , HgHPO4 , and SrHAsO 4 ). The location of the M atoms and XO 4 tetrahedra is roughly the same. There are three topologically distinct hydrogen bonds: (i) One hydrogen atom is located close to an inversion centre forming a symmetry-restricted hydrogen bond. (ii) A further short hydrogen bond has the donor and acceptor atoms not related by (average) space-group symmetry. However, the direction of the hydrogen bond (OH dipole) may be inverted and the donor and acceptor function of the oxygen atoms belonging to this hydrogen bond is interchanged. (iii) In total four O atoms are involved in the formation of the third hydrogen bond; their interactions with respect to the H atom differ. The hydrogen bond is either in or close to an edge in the MO8 coordination polyhedron; it may be bifurcated. In Sr(AsO3 OH) a super-structure avoids symmetry centres in the vicinity of all the H atoms; all H bonds have distinct donor and acceptor atoms. Complete order of the H atoms cause mono-protonated anion groups. In the other cases [XO 2 (OH)2 ] groups are necessarily formed besides [XO 3 (OH)] and [XO 4 ] groups.