Title: "Relativistic analysis of magnetoelectric crystals: extracting a new 4-dimensional P odd and T odd pseudoscalar from Cr_2 O_3 data"

Earlier, the magnetoelectric effect of chromium sesquioxide Cr_2 O_3 has been determined experimentally as a function of temperature. One put Cr_2 O_3 crystals in a static electric field and measured its magnetization or put it in a static magnetic field and measured its electric polarization. From the magnetoelectric moduli of Cr_2 O_3 we extract a 4-dimensional relativistic invariant pseudoscalar tilde{alpha}. It is temperature dependent and of the order of ~ 10^{-4}/Omega_0, with Omega_0 as vacuum impedance. We show that the new pseudoscalar is odd under parity transformation and odd under time inversion. Moreover, tilde{alpha} is for Cr_2 O_3 what Tellegen's {\it gyrator} is for two port theory, the {\it axion} field for axion electrodynamics, and the PEMC (perfect electromagnetic conductor) for electrical engineering.