Determination of Phosphorous Speciation in Sodium Aluminum Silicate Glasses Using Multinuclear (²⁷Al, ²⁹Si, ³¹P) Solid State NMR and Ab Initio Chemical Shielding Calculations

George C. Cody (cody@gl.ciw.edu)¹, Bjorn O. Mysen (mysen@gl.ciw.edu)¹, Gotthard Sághi-Szabó (gotthard@gl.ciw.edu)¹ & John A. Tossell (tossell@chem.umd.edu)²

¹ Geophysical Laboratory, 5251 Broad Branch Rd., NW, Washington DC 20015, USA

² Dept. Chemistry and Biochemistry, Univ. Maryland, College Park, College Park MD 20742, USA

A study of the structure of sodium aluminum silicate glasses (composition xNa₂O, (1-x)Al₂O₃, 0.9SiO₂) perturbed through the addition of 2 mol% P₂O₅ was performed using a combination of ²⁷Al, ²⁹Si, and ³¹P solid state NMR spectroscopy and Ab initio chemical shielding calculations (631* GIAO method). Ab initio shielding calculations of phosphorous supports the possibility of a broad range of potential phosphorous species contributing to ³¹P spectral intensity. The majority of these species are Qⁿ (n=1-4) species with variation in the Al/Si ratio of the tetrahedral ligands. Correlation of the ³¹P speciation with the Ab initio calculations narrows the total phosphorous speciation to 13 different cluster types, including Na-P type (ortho, pyro, and tripolyphosphates) and various Qⁿ(Al,Si) _p species.

A sharp cross over from Na-P dominant to Qⁿ (Al,Si)_p dominant species occurs at low Al₂O₃ content suggesting that the Qⁿ(Al,Si)_p species are energetically favored over the Na-P species. Silicon speciation predicted from the phosphorous speciation is consistent with that observed from ²⁹Si NMR spectra. Up to 8% of the silica is directly linked to PO₄ via Si-O-P linkages. In the ²⁷Al NMR spectra shifts in peak maxima towards higher frequencies as well as increases in peak half widths, due to enhanced second order quadrupolar interactions, are consistent with the presence of a significant abundance of Qⁿ(Al,Si)_p species. The combined results indicate that near the glass transition network polymerization drops at intermediate aluminum contents due to competition of Q^1-3 species over Na-P species, but rises at high aluminum content due to prevalence of Q⁴ species. Significant effects on structural properties of melts derived from such glasses, such as viscosity, may result from even these relatively low concentrations of P₂O₅.

HP Behaviour of the Heulandite-type Framework Towards Amorphization

Paola Comodi (comodip@unipg.it), Giacomo Diego Gatta (sabrina@flintstones.sct.unipg.it) & Pier Francesco Zanazzi (zanazzi@unipg.it)

Dipartimento di Scienze della Terra, Università di Perugia, Italy

The structural evolution up to 50 kbar of a natural heulandite was studied using in situ single crystal X-ray diffraction data from a DAC with glycerol as the pressure transmitting medium. Up to 37 kbar the compression of the cell parameters was linear and anisotropic with ß_a =12.1(3) 10 ^-4, ß_b=8.8(4) 10 ^-4, ß_c=8.9(4) 10 ^-4 kbar ^-1, whereas ß angle increased with a coefficient of 1.62(2) 10 ^-4 kbar ^-1. The bulk modulus, calculated as the reciprocal of the cell volume compressibility, was 330(10) kbar. The HP structural behaviour of this zeolite type was studied by comparing three refinements carried out at 0.001, 13 and 30 kbar. The pressure increase caused a deformation of the framework with a shortening of the zig-zag chain formed by the heulandite polyhedral units (FPU). The T5-T5'-T5 angles, between FPU's, decreased from 162.4° at 0.001 kbar to 156.2° at 30 kbar. At the same time the 8- and 10-membered ring channels parallel to c was squashed, principally along [010]. The channel bulk moduli, calculated by the volumes of the inscribed cylinders, were very low: 140 and 90 kbar were the bulk moduli of the 10- and 8 membered ring channels along [001] respectively and 170 kbar was that of 8-membered ring channels along [100]. Anyway the tetrahedral tilting changed not only the configuration between the FPU but also the configuration of FPU itself: the FPU bulk modulus was 830(20)kbar. Significant peak broadening, interpreted as an indication that amorphization was occurring, was observed at P higher than 37 kbar. At 41 kbar the peak disappearance hindered the lattice parameters determination. The evolution of diffraction pattern during the decompression showed that the transition to the amorphous phase was still reversible, at pressures up to at least 50 kbar.

Pressure-Induced Structural Deformation of Fluorapatite by an in situ X-ray Diffraction and Raman Spectroscopic Study

Paola Comodi (comodip@unipg.it)¹, Yu Liu (liuyu@whict.edu.cn)² & Pier Francesco Zanazzi (zanazzi@unipg.it)¹

¹ Dipartimento di Scienze della Terra, Università di Perugia, Italy

² Wuhan Institute of Chemical Technology, Wuhan, P.R. China

Pressure-induced deformation of a synthetic fluorapatite was investigated using a DAC connected with a single-crystal diffractometer and a Raman microspectrometer. Lattice parameters determined up to about 63 kbar showed an anisotropic behaviour with mean ß_a= 3.73(7) 10^-4 kbar^-1, ß_c=2.68(4) 10^-4 kbar^-1; the bulk modulus, calculated as the inverse of the mean volume compressibility, was 1000(10) kbar. Three structural refinements performed with data collected at 0.001, 27.6 and 42.6 kbar showed that Ca1, Ca2 and P polyhedral bulk moduli were 910, 780 and 2500 kbar respectively. While the tetrahedra did not change their distortion in this pressure range, both Ca1 and Ca2 become more regular with pressure. The largest reduction in the structure affected the F-channel: the channel size, expressed as the Ca2-Ca2' distance, reduced more than 2% between 0.001 and 42.6 kbar, and this explain the large reduction of the a parameter with respect to c. Both vibrations of phosphate group and lattice modes changed linearly with P. The coefficients d(nu)/dP of PO₄ group showed that the antisymmetric stretching mode ((nu)₃) and symmetric stretching mode ((nu)₁) were more sensitive to the change of P-O bond length than the bending mode ((nu)₄). HP structural data indicated that the decrease in factor group splitting of the (nu)₃ and (nu)₄ bands was generated by the reduced distortion in the local environment surrounding tetrahedra rather than by geometric changes of the tetrahedra themselves. The average Grüneisen parameters of PO₄ tetrahedra were about four times smaller than the average Grüneisen parameters associated with lattice modes. The structural refinements have shown the Ca polyhedral bulk moduli were 3-4 times smaller than the tetrahedral bulk moduli.