Developments in Ceramic Materials Research

Preface ix
crystals is discussed. At last, optical and fluorescence properties of rare - earth doped optical
fluoride ceramics is presented.
Finally, the synthesis technology and material and spectroscopic properties of translucent
oxysulfide optical ceramics of different composition doped with Nd 3+ , which can be
perspective for one micron lasing with high quantum efficiency is considered.
Chapter 4 - The solid state chemistry of polyphosphates in which the transition metal
exhibits a trivalent oxidation state is traditionally considered unusual. The great ability of the
phosphate frameworks to stabilize this oxidation state is produced for the relatively high
charge in (PO4) 3- tetrahedra that favors the formation of anionic frameworks with high degree
of mechanical, chemical, and thermal stability. The literature about this kind of compounds
shows the existence of several polymorphic forms of these phases which crystal structure has
been solved. Unfortunately, even if good crystals for X-ray structure determination were
obtained, pure phases had not been prepared. This fact has precluded carrying out any study
on the physical properties of these materials. A method based on the control of M/P ratio in
the reaction medium has allowed us the synthesis of pure phases. On this basis, the M(PO3)3
metaphosphates (M III = Ti, V, Cr, Mo and Fe) have been synthesized starting from a mixture
of the metallic oxides and (NH4)2HPO4, with different M:P ratios by using the ceramic
method.
These phases exhibit a crystal structure constructed from (MO6) octahedra linked through
infinite (PO3)∞ chains of (PO4) tetrahedra to give rise to compact three-dimensional crystal
structures. The IR spectra of these phases show the characteristic bands of the
methaphosphates with ring or chain structure. The absorption electronic spectroscopy
recordered by the diffuse reflectance method shows the presence of the spin allowed
transitions, being calculated the values of the splitting parameter of the crystalline field (Dq),
as well as, these ones of the Racah parameters (B and C). The ESR spectra of these
compounds exhibit isotropic signals, centered at values of the g-giromagnetic tensor that
depends of the nature of the metal and of its S-number of spin. The magnetic study indicates
the existence of antiferromagnetic interactions in the majority of the compounds, being
detected for several metaphosphates a weak ferromagnetic spin canting phenomenon at low
temperatures. In the case of the Ti(III) a ferromagnetic behavior has been observed. The
values of the J-exchanger parameter have been calculated using the Rushbrooke and Wood
equations for 3D-antiferromagnetic lattices with different values of the spin. In some case, the
D-zero field splitting parameter has been calculated from the simultaneously combination of
the ESR and susceptibility measurements. The neutron powder diffraction study of the
M(PO3)3 (M III = Cr, Mo and Fe) metaphosphates shows antiferromagnetic structures.
Chapter 5 - During the Romanesque and Gothic time, certain churches and mosques were
fitted with ceramic pots embedded in the walls or in the ceiling vaults. The purpose of this
action was probably related to acoustic problems, since ceramic pots were the only available
way to control and adjust the acoustic response of large rooms. Their effectiveness though is
still unknown. Research in old orthodox churches in Greece was performed to investigate the
kind of ceramic pots used, the way of embedding on the wall and finally their effect based on
most of the acoustic indexes used today for the determination of the acoustic quality in a
room. The results were compared with other research performed in Churches and Mosques in
other countries.
An analysis of the impulse response of closed spaces with and without ceramic pots,
which actually are Helmholtz resonators, showed that their effect on the acoustic quality of a