ANISOTROPIC THERMAL EXPANSION IN DIBORIDES AS A FUNCTION OF MICRO-STRAIN. Agrestini, S., di Castro, D., Sansone, M., Saini, N. L., Bianconi, A., Saccone, A., de Negri, S., Giovannini, M., & Colapietro, M. International Journal of Modern Physics B, 17(04-06):812–818, 2003.
ANISOTROPIC THERMAL EXPANSION IN DIBORIDES AS A FUNCTION OF MICRO-STRAIN [link]Paper  doi  abstract   bibtex   
Thermal expansion of diborides (AB2), with different intercalated atoms (A) is studied as a function of temperature in the range of 100-370 K by high-resolution x-ray powder diffraction. The results indicate a well defined relationship between the anisotropy of the thermal expansion and the micro-strain of the boron layers, ɛa = (a-a0)/a0 (where a0 = c/1.08 is the equilibrium a-axis for an unstrained AB2 system), determined by the atomic radius of the intercalated atoms. The thermal expansion is isotropic at ɛa = 0 (i.e., near c/a = 1.08) while the AB2 system is unstrained, and it gets anisotropic away from the equilibrium. The anisotropy increases with increasing micro-strain in both directions (positive or negative, i.e. tensile or compressive) suggesting that the micro-strain is a key variable to define the state of diborides. As a matter of fact, the MgB2 with the highest Tc shows a tensile micro-strain, ɛa = 6% and a large thermal expansion anisotropy.
@article{agrestini_anisotropic_2003,
	title = {{ANISOTROPIC} {THERMAL} {EXPANSION} {IN} {DIBORIDES} {AS} {A} {FUNCTION} {OF} {MICRO}-{STRAIN}},
	volume = {17},
	url = {http://dx.doi.org/10.1142/S0217979203016650},
	doi = {10.1142/S0217979203016650},
	abstract = {Thermal expansion of diborides (AB2), with different intercalated atoms (A) is studied as a function of temperature in the range of 100-370 K by high-resolution x-ray powder diffraction. The results indicate a well defined relationship between the anisotropy of the thermal expansion and the micro-strain of the boron layers, ɛa = (a-a0)/a0 (where a0 = c/1.08 is the equilibrium a-axis for an unstrained AB2 system), determined by the atomic radius of the intercalated atoms. The thermal expansion is isotropic at ɛa = 0 (i.e., near c/a = 1.08) while the AB2 system is unstrained, and it gets anisotropic away from the equilibrium. The anisotropy increases with increasing micro-strain in both directions (positive or negative, i.e. tensile or compressive) suggesting that the micro-strain is a key variable to define the state of diborides. As a matter of fact, the MgB2 with the highest Tc shows a tensile micro-strain, ɛa = 6\% and a large thermal expansion anisotropy.},
	number = {04-06},
	journal = {International Journal of Modern Physics B},
	author = {Agrestini, S. and di Castro, D. and Sansone, M. and Saini, N. L. and Bianconi, A. and Saccone, A. and de Negri, S. and Giovannini, M. and Colapietro, M.},
	year = {2003},
	pages = {812--818}
}

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