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1、四、四、X射线衍射射线衍射-2（X ray diffraction（XRD）近代分析实验原理（Introduction of modern analytical methods）121. XRD设备（Instrumentation used for X-ray Diffraction）Rigaku （日本理学）Smartlab 331.1 X光源（X ray source）In an X-ray tube, the high voltage maintained across the electrodes draws electrons toward a metal target (the a
2、node). X-rays are produced at the point of impact, and radiate in all directions.韧致辐射（韧致辐射（Braking radiation）：）：The electrons, accelerated by an electrical field, are suddenly slowed down when they come into contact with the target and lose some of their energy which is dispersed in the form of radi
3、ation.continuousCoolidge tubea few hundred Watts per square millimeterpower4特征辐射（特征辐射（characteristic radiation）The electrons transmit their energy to the atoms of the target and cause electronic transitions, and the emission of X photons with non-random energies E = h, equivalent to the energy gaps
4、between the different electron shells of the atom.discontinuous5旋转靶旋转靶X射线管射线管（rotating anode tube）several thousand Watts per square millimeterpowerat high speeds (2,000 to 6,000 rpm)Vacuum 10-7 Torrperformancesthe dimensions of the electron spotthe anodes surface speed同步辐射（Synchrotron radiation ）A c
5、harged particle is accelerated, the electromagnetic radiation that is emitted is called synchrotron radiation.very high intensity of the X-ray beams6多色polychromatic单色monochromatic滤波片filter单色器monochromator crystal(or monochromator)滤波片filterAbsorption：linear absorptioncoefficientthe length traveledto
6、absorb as much as possible of the unwanted peaksthickerto be as transparent as possible to the wavelength we wish to selectthinnere-d7a molybdenum anode and a zirconium filter钼靶锆滤波片1028晶体单色器（Monochromator crystals）weak9弯晶单色仪curved crystal monochromators同圆或等圆中同圆或等圆中，同弧或同弧或等弧等弧所对的所对的圆周角圆周角相等相等Radius=2
7、Rsymmetrictilting the diffracting crystal planes at an angle in relation to the single crystals surface101.2 探测器（Detector）1.2.1胶片（photographic film）The darkening of the film is related to the intensity of the X-ray beam.Linear regionsemi-quantitative measurementnot directly digitalgradual disappeara
8、nce of the useBy Rntgen111.2.2气体探测器（gas detector）the diffractometer built by the Braggs was equipped with a gas detectorNumber of ion-electron pairs.ion-electron pairs recombinethe electrons are collected without recombination雪崩one avalancheavalanche process widespreadspontaneous and permanent disch
9、arge12The different types of gas detectorsIonization chambersat voltages in the saturation areasaturation voltage depends on the intensity of the radiationHigher radiation intensity,Higher saturation voltage.heavy gas is generally usedGeiger-Mller countersAvalanche processcannot be used for quantita
10、tive intensity measurementsProportional countersHigh counting rate (107 count/s)Wavelength selectiveLong lifetimesensitivityThis system is relatively simple and is used as a mobile detector.currently the most used in X-ray diffraction131.2.3固体探测器（Solid detectors ）RadioluminescenceX radiation induced
11、 luminescenceX-ray irradiationelectrons with a high energyemission of visible photonsbombard the neighboring atoms, causing electronic transitionsDirect observation of light photonsi) Fluorescent screensIntensity of diffracted beam is much too small to yield visible spotsvisualize the primary beam f
12、or geometric adjustmentsIn X-ray diffraction, such screens are not used to detect diffracted beams, since their intensity is much too small to yield visible spots. These screens are used to visualize the primary beam while geometric adjustments are made to diffraction apparatus.14iii) Scintillation
13、detectorsX photonslight photonsElectrons and electrical signalphotocathodeii) Detectors that use CCD cameras（Charge coupled device）response time is extremely low ( 0.2 S)It is sensitive to the energy of X rays but has a poor energy resolution.15Indirect method: the imaging plateBaBrF doped with euro
14、piumlarge sizes (up to 25 cm in diameter)a resolution of up to 100 m2The main drawback is a certain remanence of the image after it has been erased.Directly measure all of the diffraction ringsphosphorescent material:16Semiconductor detectorsA voltage applied between the two faces of the semiconduct
15、or enables the userto measure the electric charge, which is proportional to the energy of the incident X photons.The main advantage of this type of detector is its energy resolution.172. 多晶X射线衍射（ X-ray Diffraction by Polycrystalline Materials）(a) Focusing diffractometers: Seeman and Bohlin diffracto
16、metersThe diffracted beams converge to the focusing circle, the diameter of which is constant, thus causing the sample-detector distance to vary with the diffraction angle.DetectorDisadvantage:18(b) Bragg-Brentano diffractometersConstant sample-detector distanceapproximate focusing systems19fixedthe
17、 sample rotates around itsaxis at a speed the detector is moving at a speed 2 along the circle, centered on the samplegoniometric circlethe most common systema major drawback:The sample is rotating, it can sometimes fall, if it is in powder form, when the angle of rotation is too high. The use of th
18、is kind of device with a heating sample holder is difficult for the same reason.20Rotate in the opposite direction with the same speed.For both of these two systems, the movements of the different elements can be either continuous or incremental.more expensiveIt is difficult to achieve a perfectly c
19、ontrolled movement of the X-ray tube, which is quite heavy.21Slitlimit the incident beams openingonly the beams with the chosen wavelength are seen by the detectorSoller slitsLimit vertical divergencehighly absorbing materialsheavy chemical elements22Without a monochromatorCan not select K1 only,Bot
20、h K1and K2 components 23Depth and irradiated volumeThe irradiated volume is constantThe penetration depth varies with the angle perfectly homogenous with respect to its thicknessrequirement24(c) Parallel geometry diffractometersThe position of the sample has little influence on the measurements, thu
21、s making it possible to produce diffraction patterns with samples that have irregular surfaces.Advantage:Disadvantage:cannot select the K1 peak, resulting in split peaks25best angular resolution263. 试样的准备（ Sample preparation）In powder or polycrystalline diffractionIt is important to have a sample wi
22、th a smooth plane surface.Normally grind the sample down to particles of about 0.002 mm to 0.005 mm cross section.homogeneous and the crystallites are randomly distributedGenerally, the powder sample is pressed into a sample holder so that we have a smooth flat surface.If we have a truly random samp
23、le, each possible reflection from a given set of h, k, l planes will have an equal number of crystallites contributing to it. 27284. 应用（Applications）Identification of an Unknown Sample by X-ray Diffraction (Hanawalt Method)the powder diffraction pattern of a substanceCharacteristic of that substance
24、, like a fingerprintsimple sumXRD patternXRD pattern of component 1XRD pattern of component nthe Hanawalt method:the d-values of the three strongest diffraction peaksEach substanceBy computerThe d spacing and intensity of the three strongest peaksIt is also important to acquire information in advanc
25、e about type of elements contained in a sample and the composition ratio of the elements contained, for example, by fluorescent X-ray analysis.Database(di,Ii)ICDD, PDF cards2930(1) If the particle size of a powder crystalline sample is larger than several 10 m, good reproducibility of the intensity
26、pattern is not always confirmed and one can frequently find a difference more than several 10% for every measurement.(2) Keep in mind the used wavelength of X-rays for measurement (for example: Cu-K or Mo-K ), because some differences may be found in the relative intensities, depending upon the wave
27、length due to anomalous dispersion factors.(3) The reverse order of the relative intensities may be found in a sample, which is easily received under the preferred orientation, like clay minerals.(4) When impurities are included in a sample, a certain shift toward the lower angle in the peak positio
28、ns is quite likely to appear. (Such shift usually becomes distinct in the peak detected at higher angles.)(5) If the goniometer center or its zero point is found to be eccentric, the position of the diffraction peaks is readily deviated from the JSPDS reference values. Badly aligned slits may also b
29、e an origin to produce similar deviation.(6) When the diffraction peak, which is not identified by the JCPDS data, is observed, careful judgment should be given by considering some factors, such as impurity contamination and presence of a solid solution phase or an ordered phase (super-lattice).Care
30、ful sample preparation and good experimental arrangement will eliminate unrelated diffraction data. Some points which should be taken into account:31Ts=500 oCTs=500 oCTs=RT3233Fig. 1 XRD patterns of (a)Bi0.74Ba0.30FeO3 and (b)Bi0.74Ba0.30Fe0.95Ti0.05O3. Thepeak marked by arrowsindicates the impurity phase ofBi2Fe4O9.34
