Thank you all for the helpful discussions.
sincerely,
Kianoush
On Jul 9, 2014, at 2:49 AM, Tim Gruene wrote:
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> Hi Colin,
>
> On 07/09/2014 09:52 AM, Colin Nave wrote:
>> [...] "If the whole diffraction process is considered as an
>> interference problem then the contributions are not confined to
>> the Bragg condition."
> Isn't this how textbooks on crystallography usually start? Drenth,
> e.g. starts with the scattering from a single electron, then builds up
> a molecule and the lattice and you find the everything except the
> Bragg peaks vanish in the noise. How does one NOT see the diffraction
> process as an interference 'problem'?
>
> Curiously,
> Tim
>
>>
>> Colin
>>
>> -----Original Message----- From: Gerard Bricogne
>> [mailto:[log in to unmask]] Sent: 09 July 2014 00:38 To: ccp4bb
>> Subject: Re: [ccp4bb] question about powder diffraction
>>
>> Dear all,
>>
>> The downstream end of this thread seems to have drifted into
>> learned considerations of spelling, so I am getting back to this
>> early reply.
>>
>> I am surprised that nobody has mentioned the role of the
>> wavelength in all this: there is no way that one can directly link
>> the first four planes in a Nickel crystal to a fixed set of 2theta
>> values. The values you quote, Kianoush, must have been observed for
>> a certain wavelength, but they would be different for another
>> wavelength. So if you want one of the powder rings to come out at
>> a 2theta of 45 degrees, adjust the wavelength accordingly so that
>> Bragg's law be satisfied for the spacing between the corresponding
>> planes.
>>
>> There also seems to be a confusion in the last question (unless I
>> have completely misunderstood it) about the orientation of a
>> crystal and the Bragg angle at which it will contribute to the
>> ring pattern of the powder it belongs to. If there is a crystal
>> oriented with some if its planes at 45 degrees from the X-ray beam,
>> that will simply determine where on each ring its diffraction spots
>> will contribute: it will have no effect on the Bragg angles of
>> those spots, that depend purely on the internal spacings between
>> atoms within the crystal, not on the orientation of the crystal. At
>> the same wavelength at which you quote the 2theta values for those
>> four rings, the crystal at 45 degrees from the beam will still have
>> its diffraction spots contribute to the rings at 44, 52, 76 and 93
>> degrees.
>>
>> Again, forgive me if I have completely misunderstood the initial
>> question.
>>
>>
>> With best wishes,
>>
>> Gerard.
>>
>> -- On Tue, Jul 08, 2014 at 04:13:59PM -0400, Edward A. Berry
>> wrote:
>>> The plane will scatter, and all atoms in the plane will scatter
>>> in phase if angle of incidence equals angle of reflection. this
>>> is how a mirror reflects. Furthermore all the parallel planes
>>> will also reflect at this angle. Trouble is the beams scattered
>>> from the different parallel planes are systematically out of
>>> phase with each other unless Bragg's law is met for that set of
>>> planes, so interference is destructive and adds up to nothing.
>>> At least that's how I understand it, eab
>>>
>>>
>>>
>>>
>>> On 07/08/2014 03:53 PM, Kianoush Sadre-Bazzaz wrote:
>>>> Hi
>>>>
>>>> If a sample of powder crystal (say Nickel) is shot with
>>>> monochromatic
>> x-rays, one will observe reflections from planes that satisfy
>> Bragg's Law. For Ni the first four planes are (111, 200, 202, 311)
>> with 2theta (44, 52, 76, 93 degrees) respectively.
>>>>
>>>> Why doesn't one observe a reflection at, say, 45 degrees?
>>>> There will be
>> a grain oriented in the powder such that x-rays reflect at 45
>> degrees and so forth. I would expect a continuum of reflections...
>>
>>>>
>>>> thanks for the insight.
>>>>
>>>> Kianoush
>>>>
>>
>
> - --
> - --
> Dr Tim Gruene
> Institut fuer anorganische Chemie
> Tammannstr. 4
> D-37077 Goettingen
>
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