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Figure 1
Schematic description of a powder scan in asymmetric diffraction conditions. The sample frame is determined by the three orthonormal vectors [{\bf X}_{\rm S}], [{\bf Y}_{\rm S}] and [{\bf Z}_{\rm S}]. The vector [{\bf Z}_{\rm S}] is normal to the surface of the sample. The scattering plane is defined by the incident wavevector [{\bf k}_{\rm inc}}] and the [{\bf Z}_{\rm S}] vector. The incidence angle [\alpha] is the angle between [{\bf k}_{\rm inc}}] and the [{\bf Y}_{\rm S}] vector lying on the surface of the sample. In asymmetric scattering geometry, the scattering vector [{\bf q}_{hkl}}] associated with the hkl reflection is in general not parallel to [{\bf Z}_{\rm S}]. The angle [\omega_{hkl}] between these two vectors, lying in the scattering plane, is equal to [({{2\theta_{hkl}} / {2}}-\alpha)], where [2\theta_{hkl}] is the Bragg angle associated with the hkl reflection. Only a small part of the diffracted intensity along the Debye–Scherrer ring (solid circle; red in the electronic version of the journal) is collected by the detector.

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