R. T. Bailey

Composition and structure of micellar calcium phosphate

Micellar calcium phosphate has the chemical composition and physicochemical properties that are consistent with it being a complex of the phosphate centres of casein with an acidic amorphous calcium phosphate. Similar acidic amorphous calcium phosphates have been prepared in the laboratory and for these, as well as for micellar calcium phosphate, the most appropriate crystalline model compound from which the short-range structure may be derived is brushite, CaHPO 4 .2H 2 O. The predicted secondary structures around sites of phosphorylation in the Ca 2+ -sensitive caseins often comprise an α-hclix-loop-α-helix motif with the sites of phosphorylation in the loop region. This motif may be important in linking the colloidal calcium phosphate with casein in native casein micelles.

Preparation of amorphous calcium-magnesium phosphates at pH 7 and characterization by x-ray absorption and fourier transform infrared spectroscopy

Amorphous calcium-magnesium phosphates were prepared by precipitation from moderately supersaturated aqueous solutions at pH 7. Chemical analysis of the samples by ion chromatography showed that up to about 50% of the phosphate ions were protonated, the proportion increasing with the magnesium to calcium ion activity ratio in the solution. When left it contact with the supernatant, the amorphous precipitates matured to form the crystalline calcium phosphate brushite (CaHPO4·2H2O). The amorphous phases were characterized by X-ray absorption spectroscopy and by Fourier transform infrared spectroscopy and their properties compared with those of a basic amorphous tricalcium phosphate precipitated at pH 10. The X-ray absorption spectra near the K edge of calcium were very similar for all samples but there were differences in the infrared spectra between the basic and the more acidic salts. In the phosphate stretching region, the main band of the more acidic materials occured at higher wavenumber and was broader. Also there was a broad band of medium intensity at about 890 cm-1 whereas there was virtually no absorption band in this region in the spectrum of the amorphous tricalcium phosphate. The acidic amorphous calcium phosphates may be useful as model compounds in describing some complex biological calcium phosphates that form near neutral pH.

The linear and nonlinear optical properties of the organic nonlinear material 4‐nitro‐4’‐methylbenzylidene aniline

The organic crystal 4‐nitro‐4’‐methylbenzylidene aniline (NMBA) was identified as a promising nonlinear material by the powder technique. The material gave a second harmonic intensity 16 times that of urea. Large single crystals of dimensions 5×3×1 cm3 were grown by the temperature lowering of a seeded supersaturated ethyl acetate solution. The principal dielectric axes were defined by orthoscopic examination. The dispersions of the refractive indices were determined to an accuracy of ±0.0015 using the minimum deviation technique and Maker fringe spacings. These dispersion curves were fitted to a Sellmeier equation which allowed the indices to be determined to ±0.0006. The nonlinear d coefficients d11, d33, d31, and d13 were evaluated at 1000, 1064, and 1300 nm using the Maker fringe technique. The coefficient d11 was over 200 times larger than potassium dihydrogen phosphate (KDP) d36. In addition, the nondiagonal coefficient d31 was similar to the phase‐matching coefficient in the organic material 3‐acet...

Amorphous calcium phosphates prepared at pH 6.5 and 6.0

Abstract An amorphous calcium phosphate (ACP) is easily prepared by precipitation from alkaline solution but at acid pH, crystalline phases normally form directly. This investigation shows that ACPs can also be prepared at acid pH provided a stabiliser is used. Magnesium ions or citrate stabilised ACPs prepared at pH 6.5 but at pH 6.0 only citrate was effective. The new ACPs differed in chemical composition and infrared spectrum from the conventional ACP but the short-range environment about calcium ions, as measured by EXAFS spectroscopy, was very similar in all the ACPs. We suggest that ACPs can form from any highly supersaturated solution provided the nucleation and growth of all crystalline phases is inhibited.

Pulsed source thermal lens. Part 1.—Theoretical analysis

The principle of operation of a pulsed source thermal lens is described and a practical system, based on a pulsed, CO2, line-tunable laser, is outlined. The time variation of the thermal lens is probed by a continuous He/Ne laser monitored on a photomultiplier tube behind a pinhole mounted on the beam axis in far field. This photomultiplier signal is fully analysed theoretically as a function of time and the physical properties of the sample gas. It is shown that this analysis enables calculation of the magnitude of the temperature at any point in the cell as a function of time. The risetime of the signal is related to the vibrational–translational relaxation time of the sample and the lifetime of the signal decay is related quantitatively to the thermal conductivity coefficient.

Surface damage of (-)2-(α-methylbenzylamino)-5-nitropyridine single crystals induced by pulsed laser radiation.

Pulsed laser-induced surface damage experiments were made on the cleaved (001) face of (-)2-(α-methylbenzylamino)-5-nitropyridine crystals at a pulse length of 25 ns FWHM. The highest single-pulse damage threshold value was 24.2 J/cm(2) at 532 nm for light polarized along the crystallographic b axis.

Organic materials for non-linear optics: inter-relationships between molecular properties, crystal structure and optical properties

An account is given of the origins, in the molecular electronic properties and crystal structure, of the large quadratic optical nonlinearities found in some organic crystals. Particular cases, in which powerful second-harmonic generation or large Pockels effects have been found, chosen to illustrate the influence of different aspects of molecular or crystal structure, are introduced. Three of these, MBANP, NMBA and DAN are described in greater detail in the light of recent experimental and theoretical work.

Thermal properties of the nonlinear optical crystal zinc tris (thiourea) sulphate

The heat capacity of crystalline zinc tris (thiourea) sulphate, has been measured in the range from 220 to 500 K by differential scanning calorimetry, and was found to obey the relationship Cp(T)=2.76×10−3 T+0.366 J g−1 K−1. Thermal expansion data have been measured in the range from 150 to 473 K. From these data, the principal thermal expansion coefficients were found to be α1=6.41×10−5 K−1, α2=4.52×10−5 K−1, and α3=−4.32×10−6 K−1. The thermal conductivity tensor of this orthorhombic crystal was calculated from values of the thermal diffusivity in the directions normal to the (100), (010), and (001) crystal planes by the laser flash method. The thermal conductivity coefficients at 295 K are k1=0.27 W m−1 K−1, k2=0.34 W m−1 K−1, and k3=0.54 W m−1 K−1.

Pulsed-source thermal lens. Part 2.—Experimental tests of the theory

The theory of the thermal lens presented in Part 1 is tested by using the decay to calculate the thermal-conductivity coefficients of Ne, Ar, Kr and Xe (48.2, 18.48, 10.05 and 6.02 W m–1 K–1, respectively) in terms of one adjustable parameter, the pulsed laser beam radius, which is found to be close to the measured value for the c.w. beam. These values agree closely with the best values currently available. The treatment of the rise of the thermal-lens signal is tested by comparison of observed with a priori calculated variation of maximum probe-laser modulation as a function of added Ar pressure. The system is shown to be useful for TEM00 laser beam mode diameter measurement and calculation of the temporal variation of the spatial distribution of temperature in a laser-irradiated cell.

Linear electro‐optic dispersion in (‐)‐2‐(α‐methylbenzylamino)‐5‐nitropyridine single crystals

The variation of the linear electro‐optic effect in (‐)‐2‐(α‐methylbenzylamino)‐5‐nitropyridine with the wavelength of the incident light at room temperature has been measured. The reduced half‐wave voltages have been found to have the values 2.1, 2.8, and 6.0 kV at 488, 514.5, and 632.8 nm respectively and the corresponding values of the linear electro‐optic coefficient have been evaluated. The interpretation of the results in terms of the structures of the molecule and the crystal is discussed. The thermal variation of the birefringence has also been investigated and the coefficient for the temperature variation of the refractive index difference is found to have the value (dΔn/dT)=9.3×10−5 K−1.