用比值法指标化X光粉晶衍射低级晶系d(A)值

TO INDEX X-RAY POWDER DIFFRACTION LINES OF LOWER SYMMETRICAL SYSTEMS(ORTHORHOMBIC,MONOCLINIC)BY MEANS OF THE RATIO METHOD

一、前言,自从德拜(Debye),谢尔(Scherrer)于1916年首先发明粉晶照相以来,一直被广泛应用于科学研究、生产实践,对于矿物研究与鉴定起了极其重要的作用。粉晶照相法虽有其局限性,如:线条的重合,低级晶系图谱复杂等,但却有其优点,如:操作简单,强度可靠,不受单晶的限制等,则是别的方法所不及的。迄今照相技术,计算方法,鉴定手段等仍在继续不断地得到有关研究人员的改进。

To index X-ray powder diffraction lines of an unknown crystal is,in general,based on the known cell constants,or on the high symmetry of crystals.The constants and A in the d-calculating formula can be cancelled by the ratio method.The experimental d(A)-values are expressed by the reciprocals of the squares of the d(A)-values,i.e.d^(*2)hkl.By dividing the rear d^(*2)hkl,by those of the first,second,third etc.,in the order of the initial descending d(A.)-values we acquire the ratio values.In order to reduce the calculation,we take first ten or twenty lines in diffraction patterns and take three dividings with the 1st,2nd and 3rd d*nkl as divisors.For isometric crystals,there is a maximum common factor(M.C.F.)in the ratio d^(*2)(hkl)n/d^(*2)(hkl)m,i.e.a*2(=d^(*2)100).For tetragonal,trigonal or hexagonal crystals,the ratios of d^(*2)koo,d^(*2)oko and d^(*2)kko are either the same as those in isometric or showing some particular values;c can be found after the determination of a and b.For orthorhombic,d^(*2)(hkl)n/d^(*2)(hkl)m=h^(2)n+k2n/(b/a)2+I2n/(c/a)2/h^(2)m+k2m(b/a)2+I2m/(c/a)2 and for monoclinic,Calculations of the ratios show that the more the approach of b/a’s and c/a’s of two crystals,the more the approach of their respective ratios of the d^(*2)hkl’s,The essential of the method is to find the b/a(=A)and c/a(=C)from the given d(A)-values by calculating and comparing the ratios of the k*2hkl(Qhkl).These ratios are arranged systematically with A,C as coordinates in order to facilitate the locating of A&C of an unknown.Sixty orthorhombic minerals of known A&C were calculated and the ratios of the nearest A and C may be discriminated,as in A=0.904,C=0.780&A=0.904,C=0.784;A=0.8919,C=0.354&A=0.983,C=0.355 and hence the method is practicable.The same and high reliabilities of both a,b,c and d(A)-values are necessary.The accuracy of calculations depends on that of a,b,c and d(A)-va-lues,found experimentally.For monoclinic crystals the angle varies from 90°to 120°.