In photographic industry there exists an ever lasting demand for photosensitive image-forming materials with increased sensitivity and image-quality. However both characteristics are often dependent on each other or inter-related which means that influencing one parameter will have its consequences for the other parameter and vice versa. This relationship can clearly be proved in testing silver halide photographic materials. So an increase of sensitivity or speed of a photographic silver halide material can be realized by an increase of the mean size of the silver halide emulsion crystals of the said material. This however normally results in a decrease of image quality. One way to solve this problem is to increase the efficiency of electron-trapping of the latent-image in the silver halide grains. This can be realized by chemical sensitization of the grains in the presence of a chemical compound which can enhance the electron-trapping efficiency as mentioned hereinbefore. Such compound may be a sulphur salt, a gold salt or a combination of both as normally used. In the recent years chemical sensitization in the presence of a labile selenium and even of a labile tellurium compound is increasingly used.
Examples of selenium compounds which were suggested for use in order to get an increased sensitivity were substituted selenourea (as described e.g. in EP-A 0 280 031, EP-A 0 458 278), selenoethers (e.g. in JP-A 2-132434, JP-A 4-271341, JP-A 6-175258, U.S. Pat. No. 5,532,120), diselenides (e.g. in EP-A 0 703 492), selenoesters (e.g. in U.S. Pat. No. 5,306,613), selenoketones (e.g. in EP-A 0 476 345), fosfineselenides (e.g. in EP-A 0 506 009), diselenides as in EP-A 0 889 354 and compounds grafted on an anchimeric long-chain group as in EP-A 0 895 121. Especially with the mentioned diselenides it has been shown that a suitable fog-speed relationship can be realized.
A variety of inorganic and organic tellurium complexes as chemically sensitizing compounds have been described as in CA 800,985 (elemental tellurium), sodium telluride and hydrogen telluride (GB 1,295,462) and sodium thiosulfatotelluride (GB 1,396,696). Specific complexes like stable palladium-tellurium complexes have been described in GB 2,316,184. Organo-tellurium compounds have been described in EP-A's 0 350 903, 0 572 662, 0 572 663, 0 573 649, 0 573 650, 0 619 515, 0 542 306, 0 661 589 and in U.S. Pat. Nos. 3,284,206; 4,076,537; 4,148,659; 4,188,218; 4,251,623; 5,215,880; 5,273,872; 5,273,874; 5,296,345, 5,340,695; 5,393,655; 5,395,745; 5,459,027; 5,561,033; 5,654,134; 5,677,120 and 5,759,760 and in DE-A's 1 96 16498, 1 96 19443, 1 96 48013 and 1 98 43081.
More recently synthesized tellurium compounds have been described in EP-A 0 809 137.
However tellurium sensitization easily tends to cause fog as a result of the depth of the electron trap(s) of the formed sensitivity speck(s) which is (are) larger than the depth of the sensitivity speck(s) created by selenium sensitization or sulphur sensitization. At the same time the stability and the gradation of a tellurium sensitized emulsion decreases if no precautions are taken. Special progress was announced by Yagihara M. in EP-A 0 585 787, describing chemical sensitization with sulphur, selenium or tellurium compounds like R.sub.1 CO--Se--X--COR.sub.2 where R.sub.1 and R.sub.2 equals alkyl, aryl or heterocycle and X equals S, Se or Te. Most of the patents dedicated to selenium sensitization like the ones mentioned hereinbefore are directed to the prevention of the described disadvantages. Moreover it is further known that sulphur or selenium sensitization carried out in the presence of a gold salt causes an increase in sensitivity but at the same time an unacceptable fog formation too. Particulary in comparison with gold-sulphur sensitization gold-selenium sensitization and even to a greater extent gold-tellurium sensitization causes a remarkable rise in fog. However until now the mechanism causing most of these effects is not fully understood yet and a lot of work has to be done in order to get fundamental improvements in the prevention of fogging or at least to get fog at an acceptable level in order to provide emulsions suitable for use in practical material applications. Accordingly there has been a strong demand for tellurium compounds offering a substantial speed increase while controlling fog to an acceptable low level. All the patents mentioned hereinbefore are to a certain extent more or less failing in disclosing a method for realizing highly sensitive silver halide emulsions with an acceptable fog level by using the tellurium compounds described therein.