214Original Research Paper

J. Jpn. Soc. Colour Mater., 93〔7〕，214–218（2020）

Novel Environment-Friendly Blue Pigments Based on Ba(TiO)Cu4(PO4)4
Hiroaki Shirai＊, Shinji Tamura＊, Akihiro Miura＊ and Nobuhito I manaka＊,†＊ Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan†Corresponding Author, E-mail: imanaka@chem.eng.osaka-u.ac.jp(Received March 31, 2020; Accepted May 28, 2020 )Abstract
The novel environment-friendly blue pigments Ba(TiO)(Cu 1－xLix) 4 (PO 4 ) 4 ( 0 ≤ x ≤ 0.15 ), have been synthesized and their color properties
investigated. By doping Li into the Cu sites, which causes distortion of the CuO 4 units, overlap of the charge transfer and d-d transition
absorption bands has been successfully controlled. Among the samples, Ba(TiO)(Cu 0.90 Li 0.10 ) 4 (PO 4 ) 4 showed the most vivid blue hue, witha blueness value (－b*) of 57.6, a value comparable with that of the commercially available CoAl 2 O 4 pigment (cobalt blue, －b* = 59.5 ).
Moreover, the hue angle of this sample (H ° = 268.7°) is close to the pure-blue hue angle (H ° = 270°).
Key-words: Ba(TiO)Cu4(PO4)4, Inorganic blue pigment, Environment-friendly, Solid solution, Divalent Copper ion

1．Introduction
Because inorganic pigments have high thermal andchemical durability with high hiding power compared to thatof organic pigments, they are widely used in paints, glazes,
and inks that are required to maintain their color for longperiods. Today, while various kinds of inorganic pigments arecommercially available, most of them contain toxic heavy
metals such as Cr, Co, Cd, and Pb, because these provide thebasis for their strong and brilliant hues. However, the use ofsuch heavy metals is becoming restricted, and it is therefore
desirable to develop alternative inorganic pigments that do
not contain toxic heavy metals. Among the colors, blue is oneof the three primary colors (together with red and yellow),
and this gives rise to a strong demand for blue pigments.
Cobalt compounds such as Co 2 SiO 4 , CoAl 2 O 4 , Co 2 SnO 4 ,and (Co,Zn) 2 SiO 4 are well known as commercially usefulblue pigments because the Co 2+ ion incorporated in thesecompounds gives them a vivid blue hue 1 ) . However,as mentioned above, since Co 2 + and its compounds areharmful to the human body and the environment, they
have been designated as hazardous substances under thePRTR (pollutant release and transfer register) system, andtheir use has been restricted in recent years 1 ). Althoughsome compounds with reduced Co 2+ ion content have been
proposed, with the aim of lowering the adverse effects of
the Co 2+ ion 2-4 ), these pigments still contain a certain levelof Co 2+, and thus do not meet the requirements for highersafety. One dark blue inorganic pigment that does not containharmful Co 2+ is the compound FeⅢ 4 [FeⅡ(CN) 6 ], the blue
color of which arises from the fact that it absorbs yellow
light due to charge transfer between Fe 2+ and Fe 3+. It has© 2020 Journal of the Japan Society of Colour Material

disadvantages, however, in the low vividness of its blue
color, and its poor thermal stability, with decomposition toiron oxide gradually occurring above 140 °C.
One source of blue color is the d-d transition of Cu 2+
in square planar or square pyramidal coordination, which
gives rise to absorption of visible light in the yellow to red
spectral region, and thus causes the compound to exhibit ablue to green color 5 ). We have therefore focused on Ba(TiO)
Cu 4 (PO 4 ) 4 as the mother crystal of a new blue inorganic
pigment, composed solely of elements harmless to thehuman body and the environment, and containing squareplanar CuO 4 units in its structure 6 ). Furthermore, the colorof Ba(TiO)Cu 4 (PO 4 ) 4 can be improved by controlling thecharge transfer and d-d transition of the metal ion, because
these are generally influenced by both the crystal structure
and the coordination environment of the metal ions, which
are easily controlled by partial substitution of the metal sites
in Ba(TiO)Cu 4 (PO 4 ) 4 by other ions having different ionicradii or valence states.
In this study, we partially replaced the Cu 2+ ions (radius0.057 nm, coordination number CN = 4 ) 7 ) in the metalsites of Ba(TiO)Cu 4 (PO 4 ) 4 with the larger Li+ ions (radius0.059 nm, CN = 4 ) 7 ) in order to intentionally distort theCuO 4 units in the structure and thereby cause a shift of theabsorption bands due to charge transfer and d-d transitions,and we investigated the color properties of the products.

2．Experimental
2.1 Materials
Ba (TiO )( Cu 1－xLi x) 4 ( PO 4 ) 4 solids were prepared by aconventional solid-state reaction method. BaCO 3 , TiO 2,CuO, Li 2 CO 3 , and (NH 4 ) 2 HPO 4 were mixed using an agate－10－