Matteo Ardit

Structural relaxation in tetrahedrally coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution

Abstract The structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1-yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm-1 with increasing cobalt amount, while the Racah B parameter is in the 744-751 cm-1 range (C ∼3375 cm-1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.

Tetrahedrally coordinated Co2+ in oxides and silicates: Effect of local environment on optical properties

Abstract The Co2+ ion in fourfold coordination provides d-d electronic transitions with the strongest optical density among oxides and silicates. For this reason, it is widely used in pigments and dyes to get blue shades detectable down to a very low cobalt concentration. Such a low-detection limit turns the Co2+ ion into a suitable probe to disclose the local ligand environment in a wide range of materials by means of optical spectroscopy. Even if extensively studied in organometallic complexes, an in-depth investigation of optical properties of Co2+ in tetrahedral coordination into oxidic structures is limited to some case-study in minerals and synthetic analogs (spinel, zincite, gahnite, willemite, calcium cobalt selenite). The present study represents an attempt to outline crystal structural (long-range metal-oxygen distances, O-T-O bond angles, and distortion parameters by XRD) and optical parameters (10Dq, Racah B and C, band splitting by EAS) in 13 samples of oxides and silicates providing a wide set of different local fourfold coordination around Co2+ added as a dopant. Subtle variations of crystal field strength and interelectronic repulsion can be appreciated in gahnite, Ca-Sr-hardystonite, Ca-Sr-Baåkermanite, willemite, Ba2MgSi2O7 melilite-related (where Co2+ substitutes Mg2+ or Zn2+ by 0.25-0.3 apfu) as well as in gehlenite and fresnoite (where Co2+ substitutes Al3+ and Ti4+, respectively, by 0.2 apfu due to charge mismatch). Results are compared with literature data about hibonite, spinel s.s., staurolite, yttrium garnets, and zincite. Spectral interpretation is not straightforward owing to the occurrence of different Co2+ bands: spin-allowed and spin-forbidden electronic transitions, two- or threefold split due to both lowering of point symmetry at the tetrahedron and spin-orbit coupling plus presumably vibronic transitions. Optical spectra vary significantly even for apparently small changes in the long-range CoO4 arrangement as measured by XRD. The expected relationship between 10Dq and the mean Co-O distance is fulfilled, but the accommodation into small AlO4 sites in gehlenite (YAG and hibonite) implies a significant structural relaxation around the Co2+ ion. The threefold splitting of the spin-allowed 4T1(F) and 4T1(P) bands can be related to the angular distortion of the CoO4 tetrahedra. Overall, changes of spectral features of tetrahedrally coordinated Co2+ can be attributed to different local arrangement of ligands with an effect correlated to the second nearest neighbors by the bond valence theory. This was disclosed contrasting 10Dq with the ratio of the observed and ideal bond valence sum for the polyhedra sharing oxygen with the Co-centered tetrahedron.

The crystal structure of Sr-hardystonite, Sr2ZnSi2O7

Abstract A synthetic strontium isomorph (Sr2ZnSi2O7) of the hardystonite structure (Ca2ZnSi2O7) was investigated with X-ray powder diffraction. The average structure (S.G. P-421m), a = 8.0007(1) Å, c = 5.1722(1) Å, was refined by Rietveld method and a RF2 = 0.0216 was achieved using 394 independent reflections. The results show that Sr-hardystonite has a melilite-type structure with no incommensurate modulations. Its geometrical and bonding features are consistent with trends defined by the other so-called normal and modulated melilite structures.

On the structural relaxation around Cr3+ along binary solid solutions

The structural relaxation occurring along binary solid solutions ( i.e . intended as the measure of the mismatch between average and local bond distances at a crystallographic site) was reappraised with the aim of revealing possible correlations of bulk properties with the departure of short-range from the long-range crystal structure. For this purpose, fourteen end-terms belonging to solid solutions, all characterized by Al 3+ ↔Cr 3+ replacement at octahedral site, were selected from literature among those of mineralogical interest ( i.e . clinopyroxene, garnet, spinel, corundum, and perovskite, using alumoniobite as a check). Along with X-ray diffraction and electronic absorption spectroscopy data used to define the structural relaxation coefficient, bulk properties were available (density and bulk modulus) or were calculated (deviation from the closest sphere packing) for each end-term. The structural relaxation coefficients exhibit a linear inverse correlation with the difference between crystal field strengths of end-members (Δ10 Dq ): the larger the deviation of local arrangement from average structure, the larger Δ10 Dq . Evidence that the structural framework, encompassing polyhedral arrangement and anion packing, affects the structural relaxation is presented and discussed. Solid solutions based on dense structures tend to relax less than those with a less compact atomic arrangement. The greater the difference in compressibility between end-members, the more the structure will behave as predicted by Vegard’s law.

Structural stability, cation ordering, and local relaxation along the AlNbO4-Al0.5Cr0.5NbO4 join

Abstract (Al1-xCrx)3+Nb5+O4 (with 0 ≤ x ≤ 0.5) compounds have been investigated through the combination of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In spite of the natural occurrence of AlTaO4, the lack of a mineral with composition AlNbO4 contrasts with the strong geochemical affinity between Nb and Ta elements. Rietveld refinements of XRPD data showed that the effective coordination numbers of the two non-equivalent octahedral sites (M1 and M2) in the AlNbO4 structure are much lower than expected, especially the one mainly occupied by Nb. This is in agreement with the very low crystal field strength values (10Dq) found by EAS for Cr3+ replacing Al at site M2. These findings imply that an unfavorable bonding situation occurs for Nb, Al, and Cr ions in the AlNbO4 structure, which can be regarded as substantially strained compared to AlTaO4, thus explaining the lack of a natural AlNbO4 isomorph. The observed long local Cr-O distances (low 10Dq) reveal that the AlNbO4 lattice is not relaxed as a consequence of the Cr-Al substitution (the relaxation coefficient ε is close to zero) and the AlNbO4 structure appears to follow the Vegard’s law. This is due to the fact that the Cr3+ for Al3+ substitution, for the limited range of solid solution (up to 0.2 apfu at site M2), does not induce any additional octahedral strain in a lattice already significantly strained.

Structural relaxation around Cr3+ at the Na(Al1-xCrx)P2O7 octahedral site: an XRPD and EAS study

Abstract The isovalent substitution of chromium for aluminium at octahedral site along the Na(Al1-xCrx)P2O7 pyrophosphate solid solution has been investigated by means of the combined application of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In agreement with the structural refinements, deconvolution of the optical spectra revealed a progressive decreasing of the crystal field strength parameter 10Dq moving toward the NaCrP2O7 end-member, meaning that the local chromium–oxygen bond distance increased along the join with the amount of chromium. The calculated structural relaxation coefficient around the substituent ion Cr3+ was ε=0.97 (i.e., very close to that predicted by the Hard Spheres model). A detailed comparison with compact crystal structures (i.e., perovskite, garnet, spinel, clinopyroxene, and corundum) as well as with other pyrophosphate compounds with II-NaMP2O7 structure-type (with M=Al, Cr, Fe, V, Ti, Mo and In) highlighted that compounds belonging to the Na(Al1-xCrx)P2O7 solid solution – characterized by a network connectivity which is basically a framework topology – undergo a structural relaxation that is confined within the first shell (i.e., occurs at the octahedral site), while P2O7 dimers act as rigid units.

Phase transitions during compression of thaumasite, Ca3Si(OH)6(CO3)(SO4)·12H2O: A high-pressure synchrotron powder X-ray diffraction study

Abstract The high-pressure behaviour of the thaumasite structure was investigated using synchrotron powder X-ray diffraction, up to 19.5 GPa. Based on Rietveld refinements, thaumasite retained the roompressure P63 space group throughout the whole investigated pressure range while the pressure dependence of the refined unit-cell parameters can be cast into three different compression regimes, each corresponding to a different thaumasite phase (th-I, th-II and th-III) related by isosymmetric phase transitions. In particular, the phase transition in the 7.40-15.02 GPa P-range (i.e. from th-II to th-III) is associated with an inversion of the axial bulk moduli which, by analogy with ettringite, can be rationalized as due to a change in the relative strengths of the iono-covalent bonds along the [Ca3Si(OH)6(H2O)12]4+ columns parallel to the c axis vs. the O-H bonds linking the columns within the ab plane. The linear inverse relationship between the low- and high-temperature data from the literature with those collected under high-pressure conditions reveals that the same bonding regime governs the anisotropic expansion and contraction of thaumasite up to ~1.4 GPa and 400 K (HP-HT stability limits of th-I phase).

The inverse high temperature/high pressure relationship in the monoclinic Ba 2 MgSi 2 O 7 melilite-related structure

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 155-16 - DOI: 10.2451/2011PM0013 Special Issue in memory of Sergio Lucchesi The inverse high temperature/high pressure relationship in the monoclinic Ba 2 MgSi 2 O 7 melilite-related structure Matteo Ardit 1 , Chiara Zanelli 2 , Michele Dondi 2 , and Giuseppe Cruciani 1,* 1 Dipartimento di Scienze della Terra, Universita di Ferrara, Italy 2 Istituto di Scienza e Tecnologia dei Materiali Ceramici (ISTEC - CNR), Faenza, Italy *Corresponding author: cru@unife.it Abstract High-temperature study of the synthetic melilite-related Ba 2 MgSi 2 O 7 (s.g. C2/c) was performed up to 1273 K. Linear thermal expansion coefficients along the unit cell edges and of the volume are α a = 8.7×10 -6 K -1 , α b = 11.0×10 -6 K -1 , α c = 8.5×10 -6 K -1 , and α V = 31.1×10 -6 K -1 , respectively, showing an anisotropic expansion behaviour characterized by α a ≈ α c O 7 reveals that the tetragonal polymorph of the barium compound (Ba 2 MgSi 2 O 7 ) should be a metastable phase favoured by high pressure conditions. Key words : melilite-related structures; melilite-type structures; “inverse relationship”; high-temperature; high-pressure; comparative crystal chemistry.

Lattice Distortion Upon Compression in Orthorhombic Perovskites: Review and Development of a Predictive Tool

Perovskites are one of the most important class of materials belonging to the category “minerals as advanced materials”. In fact, thousands of works every year are devoted to establish the countless properties of these compounds which were first discovered in 1839 as minerals in the Ural mountains and whose synthetic analogs find now application in many technological fields.