What's the difference between と思いきや and と思った


What would the difference between と思いきや and と思ったが in the following sentence:

やっと富士山の頂上に着いたと＿＿＿＿＿、まだ８合目だった。

I feel that they can both translate to "I thought~" but I think there's a
finer nuance involved.




> やっと富士山の頂上に着いたと思った、まだ８合目だった。

This would be two distinct sentences, awkwardly connected by a comma. Japanese
usually disallows joining two related clauses with no conjunction in the
middle. So you have to put something in between.

～と思いきや is an idiom that includes a conjunction (that means you cannot use it
without a following clause). It also contains a certain amount of rhetoric, so
I think the nuance would be clearer when translated like below rather than
literal one.

> やっと富士山の頂上に着いた **と思いきや** 、まだ８合目だった。  
>  _**I thought** I finally reached the top of Mt. Fuji, **where** it **in
> fact** is only the 8th station (out of ten)._

It focuses on the moment of a sudden change in situation or realization of
one's misconception. The closest alternative to this expression is ～と思ったら.

> やっと富士山の頂上に着いたと思ったら、まだ８合目だった。




[思いきや](https://www.edrdg.org/cgi-
bin/wwwjdic/wwwjdic?1MDJ%BB%D7%A4%A4%A4%AD%A4%E4) means "contrary to
expectations".

There is also an example on that page:

> あっさり断られると **思いきや** 、彼女は承諾してくれました。 I had expected an outright refusal, but to
> my surprise she agreed.

The other alternative in your question,

> やっと富士山の頂上に着いたと **おもった** 、まだ８合目だった。

doesn't sound natural since we would expect something after that, for example

> やっと富士山の頂上に着いたとおもった **のに** 、まだ８合目だった。

