The synthesis and characterisation of new inorganic pigments
The synthesis, characterisation and testing of materials as new inorganic blue, red
and infrared-reflecting black pigments is presented.
A previously unpublished route to the synthesis of microcrystalline KCoAI(P04)2 has
been developed and the structure (monoclinic e2k) of the material produced has
been verified by refinement against powder neutron diffraction and powder X-ray
diffraction. Cs2C02AI(P04)3 displays a chiral colouring of the polyhedral pollucite
network and is the first example of a transition metal-loaded material synthesised at
high temperature to do so. Refinement of the structure against powder neutron
diffraction and powder X-ray diffraction suggests that the tetrahedral sites are fully
ordered, with P(V) on one and Co(II)/AI(III) on the other. Rb2C02AI(P04)3 has
rhombohedral symmetry at room temperature (R32) and undergoes a phase transition
with increased temperature, which is complete by 500°C, yielding a chirally
decorated cubic phase akin to that of Cs2C02AI(P04)3. KCoAI(P04h,
Cs2C02AI(P04)3, and Rb2C02AI(P04)3 react to lose their intense blue colours when
fired in tableware glazes, and lack opacity when fired in glass enamels. The materials
are stable as suspensions in PVC, indicating their potential as pigments in plastics.
The ability of some chromium-doped pyrochlore materials to act as red inorganic
pigments has been explored. The material Y2Th-xCrx07-o has been synthesised with
maximum doping represented by Y2Ti1.84CrO.1607-0, which has a red-brown colour.
Y2Sn2-xCrx07-0 has been synthesised where x = 0.02, 0.04, and 0.08. These materials
are shades of pink, with colour intensity increasing with doping. The material
Y2Th-xSbx/2Crx/207-0 has been synthesised where x = 0.25, 0.5 ... 1.75, giving colours
ranging from brown to green. Refinement of the structures against powder X-ray
diffraction suggests a solid solution between Y2Th07 and YSbCr07. Upon increased
levels of doping the M-O bond distances at the mixed metal site increase, as does the
extent of trigonal distortion. This is accompanied by a reduction in distortion at the
yttrium site. Differences in the colour between these pyrochlores probably arise from
a combination of the Cr-O bond compression, and distortion at the octahedral site.
Y2Snl.92CrO.OS07-0 was tested on ceramic unglazed tiles, and also in a glass enamel,
retaining its colour and providing a glossy finish on both. On glass the material
showed less stability in a bismuth-free enamel, but retained its colour well in a
bismuth-containing enamel. Y2Til.s4Cro.1607-o lacks stability on tile glazes and glass
BhMn3FeOIO, Bi2Mn3Ti03, BhMn3.7SAlo.2S01O, DyMn20s and ErMn20S have been
synthesised as analogues to the infrared reflecting pigment Bi2Mfi401O. Of these,
BhMn3.7SAlo.2S01O offers superior reflectance. The spinel and ilmenite phases
ZnNil_xFexTi04 and Nil-xFexTi03 are reported (where x = 0.1, 0.3 ... 0.9), and
assessed to be poorer reflectors than BhMfi401O. Of most interest with regard to
infrared-reflecting black materials has been the synthesis of Bi2Mn3.7sAlo.2S01O,
which offers greater reflectance than the commercial pigment BhMfi401 (). and is also
not covered by patent law.